Thanks, that works. I guess "Custom WPS files may be located anywhere on the drive,..." as stated on the Wiki does not apply to the Android application.
Interestingly, when I have a .wps file in /sdcard/rockbox/wps/, no other WPS options show up in the Theme Settings - not even the default ones that were there before. This is in r29348-110220 for a 240x320 target.