There isn't a "rule" per se, but they are not typical in the hunter ring.
(If you spend the majority of your time in two-point, the advantage of full seat breeches is pretty much lost.)
So you won't be disqualified, or have points taken off, or anything like that. But you will stand out from the others riders because of your clothing choice.
You can by an inexpensive pair of tan or beige breeches for $40 - 50. That would be perfectly acceptable in the hunter ring. (To really fit in, you would need the $150. Tailored Sportsman breeches in the greenish beige color.) I might go that route rather than wearing the full seats if I really wanted to look the part.