depends on which build/fork you use
I use official OPL's latest Beta version and it doesn't support UDPBD, only
this OPL fork does, at least for now.
OPL's latest Beta version supports the PS2's HDD, USB, SMB, i.LINK and MX4SIO (like the latest OPL stable version) and OPL's GitHub repository main page and
this page (whose link is mentioned in the former) aren't up-to-date, which is odd.
EDIT: OPL's GitHub repository main page was finally updated and now it mentions i.LINK and MX4SIO, but
this page, whose link is mentioned in the former, still doesn't mention MX4SIO.
EDIT 2: It seems
these OPL forks support a new protocol called "MMCE", which is meant for MCE devices like MemCard PRO (only the MCP002 model), MemCard PRO2, SD2PSX, PSxMemCard, PSxMemCard Gen2, PicoMemcard+ and SD2psx. These devices are able to run PS2 games via microSD card just like MX4SIO but allow the user to save the game progress on them and may be inserted in both MC slots (MX4SIO can only be inserted on the second MC).
EDIT 3: The latest official OPL Beta versions now support + 2 TB internal HDDs/SSDs/SSHDs formatted as GPT and exFAT and they appear as a BD called "HDD (GPT/MBR)". Only OPL's GitHub repository main page mentions it, not
this page.