I do have the paid version, and I tried KEYCODE_PERIOD and KEYCODE_NUMPAD_DOT (and whatever underscore was) to no avail
Took'em from here: https://elementalx.org/button-mapper/android-key-codes/
And button mapper does work - I did map KEYCODE_LAST_CHANNEL (229) to double tap of the "i" button and as described above it does work, however with a noticeable delay (tried both variants of button mappers "custom keycode" - both work for 229 with a delay, none worked for my OTA direct entry attempts)
That's why I'm asking here if anybody here actually figured it out
ETA: couldn't succesfully get play/pause (85) to work either; that's why I tried KEYCODE_LAST_CHANNEL to achieve what I wanted