[jrockway]

You can write code to do that in your process_record_user hook, but it's probably going to be flaky. I would just use a modifier other than shift; people tend to call them "raise" and "lower" and just change the layer. That way everything is in control of QMK.

The disadvantage of using layers is that you can't press the layer-activate key on one keyboard and then press your key on another keyboard. With shift (and OS-level modifiers) you could technically do that. But it's a weird case that probably doesn't come up very often.