This firmware loading problem might be solvable with a bit of experimentation...
It always struck me as a bit odd that Parallax had development tools for multiple platforms, but somehow, Propellant.exe was wedded to windows.  Those developers had to be doing something to support not just development, but also firmware loading and testing on the platforms that they favor.
This thread reminded me of that, and after a bit of digging around, I found that Parallax has a gcc (GNU C) compiler on their downloads page, and it has extensions for working with the Propeller chip, including a separate firmware loader program!
From the manual page (liink below), it appears to be standalone.  You download the version+platform gcc compiler kit, uncompress it, and copy the propeller-load binary from its /bin and run it command-line.  No, I haven't tried it - not set up for that at the moment.
On the manual page are links to the download page.  Here's where it gets a little messy... This appears to be all the Parallax gcc releases, so you have to sift through three pages of stuff to find the download of your dreams.  
What I do know: I've pulled a few of these down and unpacked them, and the propeller-load binary sizes are different between platforms, so if you try this, you probably want to be careful about getting the platform choice right.
What I do not know: Parallax has the "Propeller 2" chip in beta, and I do not know if there would be any compatibility issues between it and our chip, so my going-in would be to suggest starting with a download of the older v0.3.4 version for your platform, which should put you in the "Propeller 1" era that I think we are in.  Then if that works, try moving up to a newer Propeller 1+2 release, if you like.
Between the wording in the doc, the relatively static releases, and the cohort using the development tool, I expect that this will work ok, perhaps with a few caveats that might need some doc.
It might be better with a wrapper of some kind to pretty propeller-load up a bit, and maybe just include it in the INDI kit somehow, but the first gate to get through would be confirming that it works as-is, so somebody on INDI and a *nix (yes, MacOS is a *nix) needs to take it for a spin. :-))  
Hope this helps...
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