I looked into shelldocs more. First, there is the same fix required for invoking the shell directly through maven-exec-plugin. However, that alone isn't sufficient to get shelldocs working on Windows. It looks like there are also going to be some requirements around translating paths using cygpath -w for the GPG commands. Even getting past that, I have some as-yet-undiagnosed error preventing it from working fully.
At this point, I think I want to proceed with committing the existing patch v002 based on Andrew's +1. I'd like to defer shelldocs and releasedocmaker to a separate issue. I don't have time to dig into those further right now, and getting the distro build working (without the site) is still a helpful step for anyone who wants to do a distro build from trunk on Windows.
I'll wait 24 hours before committing just in case anyone disagrees.