Thanks! You're right, this would be a good topic for us to cover in a subsequent handbook — in the meantime, if it's helpful, I think the way I've seen it done is pretty casual, where founders will cover expenses and then later have the company reimburse them when it has more funds. But that's more for the scenario where the bootstrapping phase is a relatively short and temporary (or at least the part of bootstrapping where you have to fund from other sources is).

If you think you might be funding from other sources for quite a while (sounds like that might be your situation), then yea, the tax considerations become more relevant and important. Another common approach is to put the money in on a safe or convertible note, though that doesn't help with pass-through taxation.