Wall incoming..! Pardon, but the question sparked a cord on something I've been pondering for a long while.
I would certainly expect it to be somewhat of a social taboo, In particular with recently deceased magi, and I would think that it is not unheard of for Wizard's Wars to be declared on the basis of someone scrying on someone else's recently departed Amicus, Covenant-, or House fellow.
Yet at the same time I can see why doing exactly that might not be prohibited in Hermetic Law. In fact it very much reflects a general tone with which I tend to read the Code of Hermes, one inspired by the school of realism in international politics. Drawing on the thoughts of, among others, Hobbes and Machiavelli it claims that when it comes to relations between suvereigns the only thing that matters is power.
On Realism in International Relations
Nothing restricts power and the use thereoff except for the power wielded by other suvereigns. Increasing one's power can only be seen as relative gains in that security is a zero-sum game where the increase in one suvereign's power can only come about through the decrease of another's. Power can - and should - be pursued and excercised without remorse. In fact not pursuing power might even be seen as immoral as not doing so will shorten your life more than it already is in this cruel world were the dog eats the dog.
No structure or organisation can happen above the suvereigns. Such structures can have no independent will of their own seperating them from the individual suvereigns = they exist only as a status quo in the power-struggles between the powers that be.
This is of course a heavily distilled version of realism, there are of course many different versions of it, just as well as there are other schools in international politics arguing persuasively why this view might be a too bleak (I'm amongst those myself).
Realism applied to the Order of Hermes
It is, however, a very interesting approach and I find that for many purposes the magi of Mythic Europe can be compared to suvereigns in a theory on international relations and in that light the limits of the Order of Hermes look rather bleak.
In a realism scenario the Order will never be able to aspire to lofty ideals as it'll never be anything but a power-balancing tool and any reform will be unthinkable. Thusly the Code is no foundation for further improvements as it can never be anything but the least standards that powerful individuals otherwise only driven by competitive self-interest could agree on. The Code is a delicate power-balance that is only adhered to as far as to avoid powers greater than one's own from annihilating you.
Equally, all aspects of the Code is geared toward keeping this fragile balance that seperates Mythic Europe from sinking into a terrible magical war. The same could be said to the various things adopted into Hermetic Law over the centuries, such as the formalisation of Cèrtamen and Wizard's War or with the rather limited tools allowed those that would work to keep the 'law' as Quaesitors. All this smacks of as minimal restrictions on the members as possible and as little delegation of power as possible, effectively restricting any form of proactive 'policing'.
At the same time several crises in the history of the Order could be interpreted as symptoms of how fragile this balance is, often requiring serious sacrifices to restore order (and not necessarily from those who've upset that balance in the first place).
The same vein of thought goes through a lot of the debates going on in the Order; perhaps most of all between the law-makers of the Order, the Guernici, and the opposing legal views held by the different groupings within that House.
Is there no option for the Order but the realisme-regime? Of course! But the politics and laws of the Order seems heavily dominated by a realistic agenda for now. And anyone who wants to change it will have to argue and work against this persuasive notion; That the Order will never become anything but a highly charged keg of [strike]black powder[/strike] ignem-imbued destruction about to blow up the moment the balance between its insanely powerful and paranoid members crumbles. The Code (and the Parma Magica) is the lowest compromise achievable to keep that power-balance somewhat un-crumbled.
And at thus it is a theme that runs through the saga I'm part of, reaching from the Hermetic politics of the day as well as a in the life-long quest of our resident Diedne-magic practicing Ex Miscallanea to find out what actually happen during the Schism War. It's dreadful and it is dramatic and it is just the stuff for some idealistic young magi to [strike]think that it can be changed[/strike] try to change it.
And why this wall of text in the context of scrying on deceased soldales?
Because the moment someone ceases to exist they become a non-identity. Dead magi - mostly that is - don't come back to vote at Tribunal proceedings nor do they threaten you with Wizard's War. Ergo there is no need to protect their finer feelings through a passage in the Code about it; the Code is about only offering the least possible protection of the finer feelings of magi that might otherwise act those feelings out, to the destruction of others. Something the dead - generally - are prohibited from doing.
Realpolitik dictates that the moment a suvereign / magus no longer wields any power, then there is no longer any need to let the Code protect him as that is dependent on the possession of power. Now, as I started this now bloated post, someone might actually care about the deceased's legacy and might make use of their power to stop it from happening, or revenge, it through Wizard's War or other more subtle political moves. This definately fits a realism worldview, the threat of retalitation, but it is not a matter for the law, as the law is not concerned with non-entities.
Edited for typos and clarity.