Disclaimer: anybody highly allergic to digital may as well ignore my post....
I think it's totally fine and practical to use the digital for proofing your LF work. I have found it to be a very valuable tool for LF slide. And no, I don't care if somebody considers me a traitor for that; anybody who's shooting 8x10 slide on an academic budget knows very well where I'm coming from People who shoot $1 film shouldn't lecture people who shoot $15 film on how to proof!
There are various adapters that allow you to mount a dslr directly on your LF camera, although because the sensor is so far recessed into the DSLR, you will need to refocus or use some focusing "stops" to go back and forth from digital to film , which is a pain. But anyway, the dslr can of course give you colour metering, histograms etc., whether you use it as a digital back or as a standalone camera.
Aside: another option that I have thought about is attaching a dslr to the WLF of a medium format camera. In that mode, it may be possible to get a digital proof and go straight to the film shot without moving anything at all. I am working on such a setup with a mamiya rz. Shooting tethered, I allege that you can get a full screen preview of the whole shot (not just a crop) plus metering info etc. and have a decent instant proof shot.
Anyway, do also consider Fuji instant film, which is available in 4x5 and is superior to polaroid in almost every respect. You can put a PA45 back on your LF camera and proof with the Fuji materials. In fact, you may find that the Fuji materials are so good that you stop thinking about the resultig prints as mere proofs. I have been extremely satisfied with fp100b in 4x5 format, as a final print.