This is a nasty conundrum that plagues lots of developers. And users as well; where you have more than one machine some may be at one level while another is at a different level.
I don't know to what extent it would complicate things for RE, but what are the chances of RE providing adapters for a 'few' levels, to be downloaded and installed by the RE client to fit the client's API level? I would suggest supporting no more than 3 levels, where one of those 'levels' is fairly early support for a beta API release. So in the current situation I would foresee 8.85, 9.00 and the next beta. For one thing that should provide a year's worth of window where a client would not be forced to upgrade. By the same token, I would recommend setting the expectation that at least a yearly upgrade will likely be necessary as a consequence of the change IB makes to the API. It's just the way things are; the client has to bite the bullet sometime.
See also Yahoo group about applying RE.