recording to Reel to Reel directly from "preamp out"

I tend to think yes, if there is a R&L channel output & maybe a switch. After all, I believe that is whats happening with a tape loop with the Monitor which is for convenience I believe. I've recorded a bunch of tapes but bever like this. As Oldhvymec says, be very careful and monitor it through the tape deck. If [possible I'd ask the mfg or whoever you bought it from. You are on untrod ground and there may be factors which we are not aware.

Yes of course you can. Just remember preamp out is variable output, the volume control will affect your signal level on the deck. You can even monitor, simply connect the monitor out from the tape deck to your power amp. Just remember to not touch the volume control while doing this or you will mess up your tape level.  

I would play around to find the preamp volume setting that will allow you to adjust the tape record level using the tape deck level with the tape control somewhere around the middle of its range. Not critical but you will probably find it easier to get a good level this way. Then just remember if you do try and monitor using the power amp the signal coming off the tape deck is driving the power amp. So it is what it is in terms of volume, you will not be able to adjust it without messing with your record level.

Just remember preamp out is variable output

Good call @millercarbon. That's something very important that I had overlooked.
A question though?

Then just remember if you do try and monitor using the power amp the signal coming off the tape deck is driving the power amp.

When using the Preamp out, doesn't that take the amp out of the system? IOW disable it? I would think so, unless using Y adapters. But I cannot see any advantage to that. He should be able to moniotor it with the tape deck headphone output. What am I missing? Thanks