I've heard of people running their turntable into a preamp to use the phono stage when their regular one didn't have one.

Are you talking receiver or a stereo preamp? Your turntable would go into a receiver's phono input, I don't see anyway to re-route it. It would also seem a tape monitor loop would be less desirable because each input ties into it, where other input paths would be direct through unless the monitor was engaged.