I used to remap caps lock to left mouse klick with karabiner, but that no longer works in Mac OS Sierra. Would it be possible to remap it in the TMK firmware?
Curious, do you do video editing? I have been considering a shuttlepro for a while, but I am not yet convinced that it will be more useful than J, K & L and some macros. Have you found it useful?
TMK is supposed to have support for "Mouse keys", yes.
I have not done it, but I think it involves setting MOUSEKEY_ENABLE ?= yes in the Makefile and then mapping the key to BTN1 (actually KC_BTN1 but the KEYMAP macro adds the prefix).