Here's one way to do what you want. You might get some ideas from this example:
In TMK, Hasu has defined a "magic" key combination. By default it's L-shift + R-shift. If you hold down both shift keys you can press another key to make something happen, including a jump to bootloader (reset).
Here's a snippet from the
TMK Keyboard GitHub page:
Magic Commands
To see help press Magic + H.
Magic key combination is LShift + RShift in many projects, but Power key on ADB converter. Magic keybind can be vary on each project, check config.h in project directory.
Following commands can be also executed with Magic + key. In console mode Magic keybind is not needed.
----- Command Help -----
c: enter console mode
d: toggle debug enable
x: toggle matrix debug
k: toggle keyboard debug
m: toggle mouse debug
v: print device version & info
t: print timer count
s: print status
e: print eeprom config
n: toggle NKRO
0/F10: switch to Layer0
1/F1: switch to Layer1
2/F2: switch to Layer2
3/F3: switch to Layer3
4/F4: switch to Layer4
PScr: power down/remote wake-up
Caps: Lock Keyboard(Child Proof)
Paus: jump to bootloader
So, as you can see, on my GH60 keyboard which is running TMK, if I hold both shift keys down (the "magic" key combo) and then press PAUSE, the keyboard controller will enter bootloader mode, just as if I had pressed the hardware reset button on the bottom of the board.