I am designing a language where a single case affix expresses both loc and gen. How should such a case be labelled?
An example would be:
- house-GEN.LOC 'in the house';
- he-GEN.LOC house-3POSS 'his house'
Given your examples, I'd recommend that you don't think of the case as GENITIVE at all but only a LOCATIVE with a metaphorical extension of property and possession.
You have a very good precedent with languages like Russian which expresses possession as location: SHE HAS A BOOK = A BOOK IS BY HER.
This sort of semantic extension is very common with cases. Plus, there's some (not implausible) argument that genitive should not be considered a case because it does not denote syntactic relationships at the clause level (but that's a discussion for another thread).
There are two possible situations I can think of, and how you'd gloss it depends on which situation it is.
The language has both locative and genitive cases, but for this word the allomorphs happen to be the same: if you can determine from context which case it is, then simply gloss it as that case. If you can't, then make a guess and note the ambiguity in a footnote.
The language has one case with both locative and genitive meanings: decide what you think is the prototypical meaning and gloss it as that. Then in your discussion of the cases explain the range of meanings it conveys. I'd probably go for the genitive in this case because in many languages it's already so diverse. Or label it Case #3. No need to confuse more people with counter-intuitive case names!