edit: Nevermind, figured it out. Somehow that always happens after I ask for help.
I installed Rockbox yesterday and am loving it so far, except the lock feature doesn't work. Select (the middle button) + Play (the top of the middle pad) at the same time do not work. Holding them both down does not work. Holding one, then the other does not work. Quickly pressing them does not work, nor does holding them for 20+ seconds.
Trying this with the default theme doesn't work, nor does it work with custom themes.
It doesn't work from the "now playing" screen, nor does it work from the menus.
I'm out of ideas. I'm training for a marathon so I need the lock feature, but don't want to go back to stock firmware. Any help? I'd even consider paying for a working lock button.
Edit: I had the buttons wrong! On my clip+ using the Release Candidate firmware, it's the middle button + the home button.
Thank you to everyone who puts time into this project, the stock firmware on my clip had multiple issues with my sd card. You saved this aging gadget from a landfill, and I will now love it and use it indefinitely.