I think understand the multi-line text field problem, and that my problem was a more basic connection issue. We have been getting by on FogBugz without logging check-ins, partly because our team is small, but also because checkins and merges to don't necessarily mean the work is done, so we haven't seen the need to trigger SCM status changes with PlasticSCM events. Having PlasticSCM branch names match issue id's has been enough for us. However, since the reason we're looking at Jira is to transcend FogBugz' limitations, it could definitely be useful to have that info available in Jira. I'll look forward to your fix for the multi-line text field to see how we can make use of checkin event info in Jira--it might be especially helpful as we expand and grow our team.
Fwiw, I was able to get around the "issues not listing" problem, by deleting the issuetrackers subdirectory in my profile, restarting PlasticSCM and re-entering the Jira connection info from scratch. There seemed to be a weird situation where Plastic client wrote Jira connection info to a AppData\Local\plastic4\issuetrackers\mycompany@cloud\... subdir, but was reading from a legacy config in AppData\Local\plastic4\issuetrackers\mycompany@local\... Once I cleared that out and started from scratch, Plastic was able to list the Jira issues with no problem.