I dunno about that, per se, but it would require some tricky thinking, and it would also depend on the radio's implementation of RDS. For example, I know some use one tuner and cache AF data until the signal is too low, and then try the other frequencies. You could probably make those work. Others seem to have two (or more?) tuners, and periodically check other frequencies in the background, and these will be a little trickier what with the N900 only having one transmitter.

Like I said, my particular radio, as long as the PI is the same (and on the N900, it would be), I can change frequency without doing anything with the RDS and the radio will find it after a couple minutes. It really depends on the radio... The longer a radio will store/cache the AF data (as opposed to only using it when it's present in the radio signal - because as soon as the fmtx changes frequency, the signal, and it's AF data, are gone), the more likely that radio is to change frequency with the N900 without doing a rescan.