Found something tantalizing. Here are the commands available for the keyboard relating to torpedoes.
Not all these commands relate to SH4: some relate to SH3 and SH5. Trial and terror is called for here.
My guess would be to set your button to Set_TDC_bearing. Exactly how the mechanics of that would work out I don't know. Ducimus made many things in the submarine clickable but isn't here to ask. But the secrets are all in TMO. Somewhere.....