Not within the Tinderbox app, but you can set an alternate shortcut via the OS
Unfortunately, you can’t — the MacOS shortcut process requires a command to have a menu item before you can add shortcuts.
The particular problem with Tinderbox using opt-tab is that it overrides MacOS’s default shortcut for quick-adding a bullet point, and neither the Tinderbox command nor the MacOS command have menu items, so the user is stuck — there’s just no way of using the bullet command, which is a shame because it’s one of the most useful editing commands. Other programs (Scrivener, Devonthink etc) use Ctl-tab to get round this problem. (Of course, I may be wrong in assuming that the editor in Tinderbox is using the same basic editing control for text…)
There are few important Tinderbox commands which aren’t on menus and suffer from the same problem: the most obvious are probably the Text Panel commands — you can turn them on and off as a whole with a shortcut, but you can’t switch between Text / Preview / HTML and have to use the mouse every time…
Third party apps like Keyboard Maestro / Keysmith can help with this second sort of problem, but not with the first (opt-tab) as far as I know.