Is there any way to determine eye relief from the info commonly found in rifle scope specs?
If they say what the eyepiece type is it might have some use. Rifle scopes should have a very wide exit pupil so they can be used in active conditions. Most rifle scopes are designed this way. Telescopes can get by with a narrow exit pupil since the observer expects to settle into a particular observing position for an extended time.
Parallax is not a problem if you buy a good scope - Nikon, Burris, Leupold, or better. Eye relief can be adjusted on most scopes, even the cheap ones. The eyepiece adjusts. It is attached by screw threads to the body of the scope, and you can change the focus by screwing it in or out. There is a lock ring so that you can secure it when you have it set properly.
The proper way to mount a scope is with the shooter present. The eyepiece adjustment controls the focus on the reticle (crosshair.) The shooter should be able to mount the rifle and immediately see a sharply focused reticle. If the reticle is fuzzy, the eyepiece needs to be adjusted. Or the guy needs glasses (this has happened to me.)
Is there a specific rifle and scope combination you are considering?