How to handle unattached ground wires in boiler fusebox/control panel (sub-panel?)?
Add a ground screw and land the ground wires there

I'd use a self-tapping ground screw (Garvin GSST or equivalent -- these are different from regular sheet-metal screws in that they use a fine-pitch (32tpi) thread instead of the 16tpi coarse pitch thread normally found on a sheet metal screw) into the back of the box to provide a grounding point, then land the loose ground wires there.  The fact that it's only grounded via the BX armor is not great, but it's better than floating altogether....