I'm replaying Ultima VIII and wanted to keymap the jump button (simultaneously clicking both the left and right mouse buttons) to something like the "J" key or the [Ctrl] key, and I wanted to keymap attacking and interacting with items (double-clicking the left mouse button) to something like the [Alt] key.

Is there anything you guys would recommend for this? I only know keymappers that map keyboard keys to other keyboard keys. Not keymappers that map mouse buttons to keyboard keys.

assuming you're playing it in DOSBox (as GOG would set it up for you), i think you can use DOSBox's built-in keymapper to do this. the DOSBox wiki should have info. if you do use the DOSBox keymapper, don't forget to delete the old / original mappings because DOSBox will happily map any one button or key to multiple different events simultaneously.

hmm. well, actually, Googling a bit suggests that vanilla DOSBox won't accept mappings to mouse clicks. apparently there are variants of DOSBox (DOSBox ECE and DOSBox-X) that might have that feature. you would have to replace the DOSBox in your GOG installation of U8 with one of these variants, and then configure its mapper.

rather than that, i guess i would turn to AutoHotKey. i think it can do what you want.

or i might reverse-engineer U8 a bit and change the binding in the executable code.

Another option is ScummVM, which has integrated Pentagram for support of Ultima VIII, and includes a keymapper. I double-checked, and it even has an explicit key action for jumping

Fair enough, it is indeed in active development, particularly due to support being added for Crusader.