Early game backpacks can be hindered by lack of llamas/cows/horses. Since other basic materials are already covered by Omnicoins, might as well throw this out there as a suggestion.

Isn't there one for cow spawn eggs?

There is already a recipe for cow spawn eggs, yes. However cows really serve no other purpose aside from being an extra early game burden as you’ll have to breed them to obtain leather.
So I agree, having a leather recipe would make sense.

From a balance standpoint, this might encourage people to buy leather for bat soul vials instead of automating them properly.

Leather can also be made from rotten flesh in the atomic reconstructor; whilst not super early game, if you need more than the 3 leather for the satchel you'll probably have access to it.

Also cows (or other animals) serve as a source of meat for Growth Medium in the late game, so it may be worth encouraging automation of breeding sooner rather than later.

Leather isn't that hard to come by. If you're having difficulty obtaining the small amounts needed at the early game (and things like the glider are very much optional), using that knife that crafts with Looting III on it and/or breeding cows are acceptable solutions.

I agree with Neeve that it would circumvent some of the automation you usually need to do. Beamsy is right regarding the AR: with rotten flesh from DML and the AR you can make a ton of leather quite easily (you usually use a Powered Spawner for Growth Medium farms though).

For these reasons I don't think we should add leather directly as an Omnicoin purchase.

Sounds good to me