Merging channels/downmixing in external hardware is probably the easier option, you might even get away with the reverse of a headphone doubler/bridged outputs but I don't know if there is anything waiting to bite me there. The "my first electronics project" audio amplifier that many go in for is also likely to be trivially tweaked into a downmixer as well.https://problemkaputt.de/fullsnes.htm#snesexpansionportextpinouts
does mention a mono audio out if you are rocking hardware. Seen a few SNES modifications to do fun video outputs, not so hard to run a wire and drill and ream (seriously pick up a handyman's reamer/tapered reamer and drill undersize) before soldering on a cable of your preferred style (female phono would be my choice but you do you) you cannibalised.
However unlike some other systems I am not seeing anything in the APU registers that you can twiddle to force a mono/downmix (for graphics on many systems you can often shove something in a vblank routine to twiddle a register that sets a graphics option every frame so you can do hacks that way, as audio registers tend to also be read/write or writeable then same idea if the option presents itself though I have only really seen it in audio stuff for volume control/manipulation).
That said I am very far from the best SNES audio hacker around here so I will leave the forest for the trees thing above.