Our troupe allows adventure experience to count to spll mastery if the player uses the spell (even once) during an adventure -- up to five points per season. Exposure experience? That's an interesting idea! Will suggest and impliment should it find favor.
As for a lab feature ... what do you propose? Seems that we get enough of a bump from those features that add to spell or item lab totals as discussed in the Covenants book.
I'm not sure, one could argue that the spell is not finished sufficiently until the end of the season, so how could you do this?
But really, it does make sense! Why not use the experience you get while inventing it for this, instead of Arts or Magic Theory.
Lab specialized in Spells applies for Lab Totals, not Study Quality.
Adventure Exp is often used for Mastery in the sagas I play. At least I do it.
using the spell no problem. Practice, teaching, exposure, adventure, books on the subject... all can be used for spell mastery. lab xp while actually inventing the spell has never been tried IMS, but I guess the troupe would have said "no".