Any mods with binaries (Windows DLLs) would need to be cross-compiled for Mac/.nix. Unless the modder provided the source for their mod, they would also have to provide the binaries. If they provided the source, you can certainly attempt to compile it yourself for your own platform.
Install Steam. A sci-fi colony sim driven by an intelligent AI storyteller. Inspired by Dwarf Fortress and Firefly. Generates stories. Explore hundreds of wild and interesting mods on the Steam Workshop. SteamOS + Linux. Minimum: OS: Windows XP; Processor: Core 2 Duo.
Mono provides compiling for Linux, OSX 10.7 (Lion) and Windows, among others, on individual platforms. I haven't checked Monos cross-compile ability yet but I have used it for other compilers. The only thing you need to consider is platform specific code but the only platform specifics I see in the context of RimWorld are buried deep or resolved by the compiler/host application (RimWorld). Mono for various platforms: Edit: Some new information shows that linux can run mono built dlls without a platform specific build. This may also be true for Macs. I wrote a quick walkthrough on how to get mods into the Mac version of Rimworld, copying it here for posterity and to make things easier on future Mac users Your mods folder is in the application itself, right click it and select 'Show package contents.'
Drop the mods you want into the mods folder. Finding the modconfig folder is a bit more involved so hopefully this makes sense: 1) Open finder 2) Select the 'Go' menu at the top, then hold down the alt button. You should notice a new item pop up - 'Library' - click it on it while holding down alt. 3) You can let go of alt now 4) Look for a folder named 'Application Support' in the library folder, open it 5) Scroll down to 'Rimworld,' open it, and voila - config, save and world folders!
Tip: OSX really does not like merging folders, so if you use any mods or modpacks with instructions to merge/replace - ignore it. OSX will always replace the entire original folder with the new one. You'll need to figure out what is being merged/added and manually drop all of it in. I can't believe I never googled how to merge folders on OSX.