I've got a Sapica card, and apparently it's only useful in Sapporo.
Is it possible to get a refund for Sapica cards outside of Hokkaido, such as in a nation-wide convenience store chain?
No. Japanese transportation IC cards in general can only be returned in person at an office of the organisation that sold them. Sapica is sold by the transportation bureau of Sapporo city, so it can only be returned there. (Here is the official page in Japanese stating that, for those interested.)