Ok, been thinking about this topic the last few days. First I want to point out that I have no evidence of this, but my gut says the Mysteries Revised may shed insight into this subject that we currently lack. That insight could easily resolve that matter. But lacking the book, we need to go with canon material.
I'm looking at the core book and Calebais.
My arguement is simple. There is a difference between a spirit and a ghost. Spirits are 'easy' to manipulate, ghosts are not.
Pg 184 of Ars 5 defines spirits. It is not a simple one sentence definition, but rather an explanation that spirits is a term that encompasses a wide range of creatures that are 'essentially spiritual'. That may be core definition, a spirit is a creature that is 'essentially spiritual.' Slightly self referential, but I hope for this discussion it is adequate.
Calebais pg 35 explains ghosts. Again it doesn't provide a simple sentence definition, but I think one can be derived. Ghosts are, "disembodied spirits still confined to earth because of their actions in life, or because of a supernatural desire to complete a task left unfinished."
Now that 'core' definition does not detail or bind a ghost to a particular site. However it does detail that what seperates a ghost from a spirit is motive. Ghosts possess 'a supernatural desire to complete a task left unfinished."
I argue that this desire to complete a task may be the thing that binds a ghost to a particular location. This is because the desire to complete a task is typically tied to a specific location. The location where the task was not completed. The ghost is where they are suppose to be, they simply lack the ability to make peace at this location.
Because of this, I think it would be very difficult to force a ghost to leave their haunt and complete an assignment such as deliver a message. Now I want to be clear, does it break a Hermetic Limit? No. So is it possible? Yes. But the focus of a ghost is so narrow, any sort of spell that would compell a ghost to do anything besides complete it's specific task should meet a great deal of resistance.
So let us work with the premise that all a Hermetic Magus need do is over come the Might of a Ghost to compell it do to something other than it's specific task.
If RegoMentum > than Might, the ghost is compelled.
I suggest that twice a day (sunrise and sunset) the ghost spend one point of might per magnitude of the spell it is compelled by trying to over come the spell that is preventing it from completeing it's chosen task.
I think this sort of focused fight makes sense for ghosts. It also suggest that a ghost would essentially destroy itself trying fight the magus that is compelling it.
Now this sort of destroys the usefullness of the spells detailed. The end?
First, it may be possible to find a ghost that travels. Perhaps the ghost of a Red Cap or a messenger. Such a ghost would be compelled by it's nature to deliver messages. This sort of ghost may be eager to deliver the message.
Second, I've tried to establish that there is a difference between a ghost and a spirit. Because a spirit lacks the determined focus of a ghost, I think that once one over comes them, that a spirit would be compelled to complete the task as detailed in the spells.
Perhaps it's simply symantics, but in my opinion both of the above spells would work quite well on 'spirits' and very poorly on 'ghosts'.
Ars 4 introduced spirits of artiface (sp?) and if these sort of spirits return in Mysteries Revised or a Hedgey Book, then I think we will gain more insight into spirits and the powers that can be used to compell them. I've always had a soft spot for necromatic magic, but that soft spot doesn't include depowering ghosts. I think ghosts should be tough because of their narrow focus.