I'm working on getting a mac so I can better support that platform. Right now the problem is that symlinks break. It's fixable if you download nw.js and copy the correct files over, but its impossible for me to do on my own right now until I have a mac.
Ah! Yeah, I'm familiar with that problem (py2exe has the same issue). The simplest way you can deal with it is making a .tar.gz instead of a .zip. (I believe the recommended fix is to always produce a `.dmg`, but I suspect your non-mac build environment would make that unduly challenging.)
I've heard of "rent-a-mac" sort of services. Bitsy Fire was definitely a hobby project, but my currently in-development title, Visual Out, is aiming to be a full commercial release and I'm totally willing to invest in a (cheaper, refurbished) mac for it and future projects. =]