This does appear to work under light testing; after applying and massaging the patch I was able to add a default route to an 8.3-i386 jail on a 9-STABLE-amd64 host. I put the updated-for-9-STABLE patch here: http://pastebin.com/iThJpMTe (The referenced patch is by Sergey Kandaurov, not me, and I am not claiming any credit for it.)