For the curious; it is possible to use Google's AOSP 'repo' tool https://gerrit.googlesource.com/git-repo/ along with a post-sync Python script to quickly and easily pull down an OpenWrt development workspace.
I've got (very) rough versions in these repos:
I use the GitHub mirrors because as a Canadian user, access to git.openwrt.org is very slow, but using the official git mirrors instead is just an edit away.
Over time I hope to make these more generic (so more useful to folks besides me), as well as cleaner and better.
In the meantime, feel free to take a look at what is there.