I'd love to see this implemented - having a click track, that'd fire on rows per beat + rows per measure would be excellent - especially if were an option that could be assigned a keyboard shortcut, to toggle the metronome playback on/off

r22669 / OpenMPT 1.32.00.39 implements a customziable metronome that can also be toggled via a keyboard shortcut.

Test builds will be available from https://builds.openmpt.org/ within a few hours. Happy new year. ;)

One thing you might run into: I guess it would have been nice if the metronome was automatically disabled when record mode is off. However, while record mode is in theory a global setting like the metronome, historically it has always been possible to toggle record mode independently for all open pattern editors. Hence we cannot disable the metronome very easily based on the global record setting, and it's probably not a good idea either to remove this deliberate feature of being able to only put some open modules (or just one of multiple pattern editors of the same module) into record mode.