Ah, no, should have looked more closely - it looks like it's compiled against 2.9.2, and anyway test-interface is probably the more likely culprit. What's particularly galling about this is:
(a) the "bad"-looking test interface jar is not on the test:full-classpath - so full-classpath is a misnomer
(b) another version of it, presumably the correct version which specs2 already depends on anyway, IS on the full-classpath!
So it looks like I just need to figure out how to rip out the offending duplicate jar for test-interface.