What is the largest denomination you want? If using metals, up to 10 pawns seems doable with a coin made of gold.
I had envisaged up to a Queen of Vis; but honestly that is incredibly impractical. I think including brass dust or granules for denominations up to five Pawns is probably where I may leave it. The smallest denomination is probably a fifth or a quarter of a Pawn, useful in light of the revaluing the text economy thread.
Why not just use standard vis? I'm actually curious about what in your scenario requires the usage of these coins instead of going with the standard, and the answer could very well help to flesh out the method. Is this a specific need of a particular covenant in your saga? A change of assumptions for the whole of the Order? A need that arose from a particular problem your magi has encountered?
Mostly I am just trying to sanity-check this idea. I want to be sure this does not do anything egregiously out-of-bounds.
I think it might have uses with Companions (and perhaps exceptional Grogs) to occasionally pay them a pawn (or a fractional pawn) for particularly valuable / magic related services -- those could be accumulated so that they could then barter for Longevity Potions, Fertility magic rituals, minor invested items, etc. They might be nice flavor at Hermetic faires; or in auctions & exchanges of various flavors of Vis. This is quite aside from the fractional-pawns discussed in the revalued book trade, where fractional-pawn coins might be the only reasonable way to handle fractional pawns.