Is the study of the volunteer sector inappropriate for a labour economist?
I would agree that volunteered labor is a utility enhancing activity. Seen as a good, its price could be the opportunity cost, i.e. the wages foregone (for not working for pay). But then this is not really different from the concept of "leisure", since "leisure" in economics does not mean "fun and games", but rather "time spent not working for pay".

An interesting extension could be the potential value of volunteered labor as professional experience towards future employment for pay (in line with Arrow's "Learning by doing" efficiency concept). Then the issue becomes intertemporal, and also, income considerations arise: I want to volunteer, it generates utility directly for me, but also, I'd rather volunteer and build up professional experience that may lead to higher wages tomorrow, than try to work now and have lower pay due to smaller experience.