This project continues to intrigue me.
If you find brass, will you be able to find primers? If no primers, could the brass be modified to accept 209 shotgun primers?
What about machining an insert to allow the use of necked up 50 BMG brass? The brass is about 1/2" short so there'd be a bit of freebore. It might not be feasible if there is not enough difference between the outside body dimensions of the two rounds though. If not, an insert could be made for just the base, and the brass would fireform out to the chamber dimensions. The insert would then become part of the round, to be reused when the brass was discarded.
Resizing is a problem no matter what route you take. Neck sizers are easy to contrive, body sizers less so. If you keep the pressure down, you can get several neck sizes before the body becomes an issue.