I'm not sure about the MC-Part (if you can store themes, BG and such there...).
But you can load imiges from every device, which is 'activated' in your config (USB, SMB, HDD).
USB-Root --> put the folders here
SMB-Sharename/PS2SMB (for example) --> Put folders in here
internal PS2-HDD --> +OPL-Partition --> put the folders in this partition
...if I remember correctly.
So I guess the only thing from OPL which you need on your MC, are the configs... (and maybe one theme/skin on MC, if you do not have the device where your skin is located at, connected on start-up).
PS2 V7/DMS3 V2 (FW:2.4b7); Seagate Baracuda 200GB
PS2 V7/CC1.0 (FW:34 hacked v2 BM:2.1.6); Maxtor DiamondMAX9 PLUS 160GB
PS2 SCPH-30004R; NoMod+NoLaser
3xSony MC 8MB
MAX/Datel 16MB with Boot-CD
Custom FMCB 1.8b+ Beta-Build, my AIO 0.5, Sony&xRhino-Linux