Optionally Don't Mute Pre-Send

Could you do this in the routing matrix? It is a workaround, but it should work. You can just remove the hardware output on the master and create a direct hardware output on the track you want to solo.

Haven't tried it, so I just throw that in: Maybe it depends on the Preferences setting "Do not process muted tracks..." -Data

Is it Tracker time?

Since we're not in the Tracker yet: +1 He's right. When recording live musicians on multiple tracks, or even overdubbing, it's very convenient to solo with sends, and not interfere with the musician's cue mixes. Most of us in similar situations work first through an analogue mixer before the DAW. If accomplishing this for now means rerouting through the record matrix to make a separate feed as Nicholas has suggested in the General Discussion Forum (as if to create a Control Room monitor), then this FR in the Tracker needs a lot of support:

Just saw it in the Tracker: Please support.

IMHO the problem is buried deeper than muting pre or post sends when soloing. Solo (in it's various ways like PFL, AFL etc.) should (as in any console) only affect a dedicated controlroom output AND NOTHING ELSE. Mute should affect any output of a track (preferably optional pre- or post sends) and should be overridden by solo only in the controlroom output (and only there, nowhere else!). REAPER's concept regarding solo is more than strange.