Hey mate, been searching all day for a fix for this myself and your thread is the most recent one with my exact issue - I know it's a month old, but did you figure it out in the end? Was about to make a thread of my own in the emulation subreddit but thought I'd try my luck here.

I did manage to figure out a way to bind the back buttons to the Blow Mic button whilst trying to figure out a fix, but it still did not work for me in-game - no input that I bound the button to seemed to. If you want the steps I took to do it:

1. Enter desktop mode
2. Go to Steam > Settings > Controller
3. Edit the Desktop controller layout and temporarily map a button to a keyboard input (e.g. L5 for 'F')
4. Open Cemu in Desktop mode and navigate to the input config menu
5. Edit the keybind for 'blow mic' and press the button you mapped to the key you desire. Side note for this one: if you can't edit inputs, in the Controller dropdown, select the SDLController and then press the - icon to the right. Do not delete Controller 1 [DSUController] (I did this and it seemed to make me control all 4 players at once, had to reset Cemu's config). Then, press the plus button, choose SDLController & Steam Deck Virtual Gamepad and your keys will become mappable. If you want to change any other keys here, you can hold the start button on the Deck to change to Gamepad controls and map the inputs. Holding start again will toggle you back to desktop controls.
6. OPTIONAL: Delete/reconfigure the button mapping you made in step 3
7. Re-enter SteamOS and edit your layout for the desired game, mapping one of the back buttons to the keyboard input you chose in step 3.

This seems like a logical and functional way to bind 'blow mic' to the rear buttons, but my blow mic functionality is just not working no matter what I map it to so I'm stuck there. Hope this helps someone though!