A temporary business visit under visa waiver or one of the B categories does not, as far as I'm aware, allow you to "work" except under certain circumstances.

Meetings, sales calls, repairs, market research, and similar activities are not considered "work" and are allowed. It would need to be investigated whether R&D (which, effectively, is what you're doing when working on a YC project) is allowed.

If you're working in the country already it's unlikely you can use that. I tried to switch to a tourist visa after leaving my job (was on a TN). Denied - they said that I failed to reestablish residency in the foreign country.

That's a very small list of countries. Large, no!