Some devices without a hold switch use a software hold function. The clip+ is one such device. In the keymap file (apps/keymaps/keymap-clip.c), the following line is included in the WPS section:
{ ACTION_STD_KEYLOCK, BUTTON_HOME|BUTTON_SELECT, BUTTON_NONE },
This will use the key combination of Home and Select to activate the software hold (KEYLOCK) while in the while playing screen.
Similar changes could be made to the mr500 keymap file. You would need a build environment to be able compile the source code with your changes.