Great initiative, but to be honest I'm having trouble seeing the upside for the freelancer.  Being "underbanked" isn't really a thing anymore with the digitalization of fiat.  Most of us do our banking online, and hardly ever visit a brick and mortar bank unless we have to.  And if they're underbanked for some other reason, that's likely going to affect whether or not you can be their bank (or open an account on their behalf, as the case may be.)

If I was to invoice someone who couldn't pay me in bitcoin, and therefor had to accept fiat but wanted bitcoin, I would just accept the fiat and convert it myself.  I understand that the middle man doesn't want to be left out, but in previous such circumstances I've been doing that all along and I don't see what benefit the middle man can provide me.  Unless there's something else that I'm not picking up on, like tax evasion or some such thing, but there's going to be a banking trail regardless and that would provide risk for you, and not prevent the client from facing the same risk they would incur if that were to be their motivating factor.