Worked for me--I ended up putting the 'new' version of uuirt.dll from the SageTV thread linked above in both System32 and in SysWOW64, since a search revealed a copy of the old DLL in both places. (It also showed up in the System32\DriverStore hierarchy, but good ol' SYSTEM wouldn't let me copy it to that location, and I didn't feel like fighting it.)
In limited testing, it looks like MC64bit once again recognizes my old USB-UIRT, and the few remote commands I tried worked as I expected.
Thanks for the pointers! Check another item off my To-Do list and saved me $20 in buying a new IR transceiver!
brad