I don't think a rule change is required for a "Friendly" challenge, after all we all belong to BFTA
For the dates Cliffy suggested, CSFTA turn up an shoot at Newbury as per our CSFTA winter league date. The SEFTA guys turn up as visitors and shoot. For the SEFTA home leg, which is one of their WL datess, the CSFTA guys are just visitors.
The regional winners would be from the combined score cards from both dates. The emphasis should be starting up some new competition