No OS comes with RAR support builtin. That's because it is a non-free format.
The only reason ZIP as a format still exists is because it is ubiquitous and you are guaranteed that anyone can open it.
Using RAR downloads defeats that purpose, and in that case why not just rewrite Sigil to use RAR plugins?
If anything, bzip2 or xz (backed by tar) should be used, since
at least both OSX and Linux have builtin support.
Windows users would have to fend for themselves either way...
AFAIK, Google doesn't mess around with either of those compression formats either.
I really don't understand why you are pushing so hard for the adoption of RAR, versus using a different download host.