Oh! What a bad break, Steve -- literally!
I think this is a classic case of rust-out caused by water getting inside the chainstay bridge and then remaining there.
The good news is, replacing a chainstay bridge is very easy for a framebuilder and the cost for such a repair is modest. The heat of brazing will toast the paint on the stays, requiring a local repaint of the area or a complete respray.
My one concern would be whether the rust has migrated into the chainstays. It is possible, so it might pay to probe gently with the end of a screwdriver, now the bridge is half open.
Even if the chainstays have been compromised, it is possible to make a patch -- essentially a lug overlay, as with reinforced bottle bosses -- and then braze a new bridge to that. I've done so myself on past bikes, and the repair came out well. If it were me, I'd definitely save the frame by going with a replacement bridge with or without reinforcement.
Such a pity this became evident so soon after your cracking-great orange repaint, Steve. If only it had been evident before the respray, it could have been replaced then, but...well, such is Life sometimes.
Best,
Dan.