Is there a specific switchable stereo to mono Jack you're using?
I've been thinking about modifying the output in my 70's Japanese Rickenbacker copy.
It has a stereo output socket, but is wired up in mono.
I've considered wiring it in stereo, and always using a stereo lead to a DIY Ric-O-Sound box, with stereo input, mono neck output, mono bridge output, and summed mono output.
I've also considered adding either a switch for mono / stereo output (aesthetically my least favourite option), or a push / pull pot if possible (one on bridge tone for the cap bypass, and one for mono / stereo).
Another possible solution would be replacing the 3 way pickup toggle switch, with a 4 way, with one position wired for stereo (if possible).
Ideally I was hoping a switchable output Jack might be available, but I haven't come across anything yet.
How are you solving the problem?