I discovered an interesting potential rules loophole.
Every time you open a talisman for further enchantment, you can activate one of the Shape and Material bonuses for spell casting.
Major Philosophic Alchemy lets me open a device as a 7-day distraction each season.
Theoretically, this means that I could add an attunement every season as a 7-day distraction, limited by the maximum capacity of the Talisman and my skill as an Alchemist. I would actually be able to activate MORE attunements, the worse I am at Alchemy, suggesting that I have problems with up to 19 days distraction (Alchemy fails after 20 days distraction).
Given the minimal investiture of time, I could see a magus intentionally hampering his Alchemical efforts to open the device to allow for the activation of a carefully thought out series of bonuses.
Is this kosher? Would you let this happen in your saga?
Don't you just open the item the one time, when you put the initial vis into it and prepare it for future enchantments? Ah. Inset on p. 96 of the main rulebook. Invested items "must be opened to enchantments." Lesser enchantments "need not be opened." The next step (in the same summary inset) says "Open item for enchantment (Invested Items only)" and goes on about vis cost, S&M bonus, MTx2 limit.
So, it looks to me like "opening" the item refers to the initial procedure. The subsequent steps (which take mere mortals a whole season or more) is "investing" the effect(s).
But...the talisman rules on p. 98 get wonky, here. Says you can "open your talisman to one kind of magic attunement...every time you prepare it for enchantment or instill an effect."
I would say that the opening a device as a seven-day distraction only applies for the initial putting-vis-in-the-item phase, while subsequent "opening" is the actual putting magical whiz-bang into the talisman, which takes a whole season plus.
I'm inclined to say it doesn't apply to talismans, beyond the initial opening. Philosophical Alchemy (PA) allows the magus to do an end-run around the Magic Theory limits and charge a device and then continue opening it for enchantment in a subsequent season. Also, note accumulating Vis into a device takes a season of time, just as if you were using PA to extract Vis from an aura, which you treat as a distraction from other work, while doing other lab activities or study. Furthermore, the need for doing this for a talisman is less necessary, unless your MT limit is less than your CrVi extraction total.
IMS I'd be inclined to disallow you the benefits of the talisman while you are charging it thusly, and I'd also tend to think that you didn't actually do enough with your talisman to create an attunement with the trickle charging PA allows.
I don't think it is a loophole, at all. A magus without PA could add a single additional pawn after it is attuned as his talisman. However they are spending that season drawing out the magical attributes. I envision the PA leaving it under some alchemical apparatus collecting the Vis in a trickle as it is distilled from the aura. The PA is not actively doing anything to/ with the talisman. It is a passive process. Set it and forget it, checking on it in frequently. Get the Vis, sure. Get an attune meant? No. It takes a season of time, and the magus is busy doing something else.