Is there anything to stop people uploading PPMs to CPAN?

I don't know. What would the PAUSE server and indexer make of a ppm package ?
Is there a way of just sticking a file on CPAN such that PAUSE doesn't try to do the things it does when a source tarball is uploaded ?

Even if it could be done, I wouldn't. I regard CPAN as a source repository, and would consider that I had "breached faith" if I started putting binaries there. (Yeah, I know ... it's a rather fuddy-duddy antiquated reason, and not a very good one. Btw, I've no objection to someone else putting Windows binaries on CPAN - it's just something that I myself am not currently prepared to do, even if it is possible.)

Besides, I already have access to the Uwinnipeg repo - so I could stick it on there if I wanted to. But as (my perception of) my access to that repo isn't one of "anything goes, open slather", I don't usually put anything there unless:
a) it's difficult/tedious to build, and
b) I'm prepared to update it regularly.
I must admit that there are some modules that I've placed on the Uwinnipeg repo that don't get updated as regularly as they should :-) ... but that's no reason to add to the list.

Of course, offering to email a ppm package is a long way from the ideal situation - especially when someone digs up the offer 6 months down the track, by which time the package has been lost and needs to be re-built (as has happened to me more than once).
It would certainly be good if there was some place to which we could just upload ppm packages willy-nilly.

Cheers,
Rob