Yes, I run PD and RME Multiface with Jack also. PD/RME doesn't like ALSA, but seems to be fine using ALSA's OSS emulation. This might be worth a try if you didn't compile PD with --enable-jack.
at the iem we are using the RME/hammerfall/hdsp/multiface cards with plain alsa (since jack turned out to be far too unreliable).
but i'm curious about your experience with jack ... is it a problem in the architecture of jack or the jack part of pd, that you have been thinking about?
cheers ... tim