Perhaps an announcement along the lines of "people who want to meet staff face to face to discuss our products will be able to find us at the RISC OS roadshows in... on..."?
Speaking as someone on the MPro mailing list, I (obviously) already know about the roadshow. However, I probably wouldn't be offended by such an announcement sent to the list (especially if, say, programmers will be present to talk to, or you'll be showing a beta of the next version), so long as it remains a special case, and not something that happens for every RISC OS-related event.
 or you make it specifically relevant in some other way; special offers for existing users who turn up, perhaps? (Not that I can make it anyway