I’m not that worried about JIRA mis-use. I think things will work out automatically, as people tend to use tools that they like to report bugs:
People that will make use of the opportunity to report in JIRA directly will be used to working with bugtrackers: these people tend to give you more valid (or less crappy) bug reports. Us giving them access to JIRA indirectly indicates that we take them serious. In my former job, co-workers (developers) always frowned on having to report a bug in a forum. I’m sure that there are developers out there (I’ve talked to a couple of them) that won’t report bugs to the forum. For some reason, having to sign up for a community is more of a burden to them than to sign up for JIRA (something they’re more comfortable with)
The type of people that is likely to give you inaccurate, or plain false bug reports, are often more comfortable with a forum-type of thingy. They’ll keep reporting bugs there.
In any case, both JIRA and SBS would be complementary.