This doesn't look too difficult to do (definitely not a "wizard" level task), and I think there will be more work in setting up tests, etc than actually writing the code. A good learning opportunity so if you've got the time go ahead and do it yourself.
If you end up doing it yourself, I'll volunteer to do a quick review.
Courage! You can do it! It would also be a great learning opportunity since you have the other 2 libs to use as an example. You should go for it and then put it up for code reviews before using it. Seriously! Give it a shot and then ask HN to review it in another Ask HN.
I'm a sole developer in pyjamas coding in the lounge room trying to come up with an idea that will make an income when I'm not doing my day job to pay the bills, in case you're asking if I am a rich company trying to get free development services. I'm happy to reciprocate technical services if what I know is of value. I write open source code too so I'm contributing although not to the libgit2 project.
Same as many other developers here on HN I suspect. Coding in the hope of building something people want to use.
So, you're looking for somebody else to do at least some of the technical work for a passive income project, without some sort of profit-sharing arrangement or transparency about that fact (until I brought this up)?
Truly rich companies are more likely just to pay for talented developers or services than small folks--see also aquihires.
Anyways, enough of the business/philosophy stuff...what are you looking to gain by using PG as a backing store?
I had a quick look and agreed for a learning exercise this would be fantastic and given you have two existing examples to compare with it would be possible to work it out and learn some C at the same time and even if you did not know C; You would be able to hack it together without much brain taxing.
The PostgreSQL developers team lives on the pgsql-hackers mailing list (http://www.postgresql.org/list/pgsql-hackers/) which is appropriate for discussion of current development issues, problems and bugs, and PROPOSED NEW FEATURES. =)
But it's not a postgresql feature, it's a libgit feature. That said, you might still get a review out of it, but it's certainly not the obviously correct place.