Why not just hold down one of the Shift keys while typing? That should be faster, considerably once underscores are involved, requiring only slight adjustments to typing technique, to use a ring finger where you would normally use a pinky.
(This is a genuine question, because I’m confused why one wouldn’t do it that way—I think it probably never even occurred to me to use Caps Lock for such a purpose, given that the modeless Shift key was right there.)
I find it mildly uncomfortable to hold shift while typing. And as there is a key for that purpose, which is in the same place on all keyboards, it seems more straightforward for me to use it for its intended purpose rather than something else. I’m not even sure what I would remap it to.
