I did some further testing of wiiflow 745 and here's what happened.
Wiiflow dol in SD/apps
Wiiflow folder in USB1
Data on usb=yes
Loads music from USB1/wiiflow/music
creates and loads cache on USB/wiiflow/cache
Loads plugins from USB1/wiiflow/plugins
Does not load source menu icons from USB1/wiiflow/source_menu
loads themes from USB1/wiiflow/themes, but they're broken. Missing buttons on two themes I tried, no source menu icons at all from USB1/ wiiflow/themes/abzcarbonik
only Source icons 3,4,7,8,13,14,15,17,18,19 from USB1/wiiflow/themes/8bit playground. same themes work just fine from sd
won't load boxcovers from USB1/wiiflow/boxcovers
Finally and my only real goal, emu plugins load files fine from sd, but not from USB, mplayer is just the opposite
Gonna try 753 in a little bit