You seem to be assuming that there will be no cross-linking between the blog and the main site.
If you cross-link normally, the PR and every other benefit will flow just fine. The worst you can expect is that it might take a bit longer for the blog on a separate subdomain to "find its feet" SEO-wise, but even that is a "maybe" and it can be overcome with savvy link development.
As for the reverse proxy stuff, why make life more complicated than it needs to be? That's just a recipe for trouble. To me it would make a lot more sense to invest the energy into link development.