This has been requested a few times but I'm not sure staff have ever commented on whether they're interested.
I did wrote a custom app that can do this plus a few other things I was looking for: [RELEASE] Lights on Motion Plus (dim before off, remember individual bulb states, etc.). Maybe you'll find it useful for you too. (Edit: I just realized the this isn't quite what you want--it will still turn them off, though if you code you could probably modify it.)
Another thing you could consider is using Motion Lighting or even Simple Lighting to turn on your lights and use your rule to handle dimming with inactivity. You likely won't notice the extra milliseconds in that case, but you'll still get the speedier "on"/level change actions.
