iSyncr will maintain synched playlist, but I don't think that's what you're asking it do to. In other words, if you add/delete songs in iTunes itself, there is an option in iSyncr to 'Delete Abandoned Songs' which will do just that.
You'd need a file manager like Astro or a music player like PowerAMP which allows for selecting & deleting the files to maintain it on the phone. That or hook up the phone via USB and maintain accordingly.
There may be other sync apps out there that do allow for this, but I've always used iSyncr since it works so well so I can't suggest any that might.
You could always create a dynamic smart playlist to achieve this automatically. For example I have one that just holds any music added to iTunes within the past 21 days (I add a lot of music regularly). This playlist always has fresh stuff and updates on it's own via iTunes/iSyncr.
Good luck!