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So it's wet and winter, and my shop is one big open space without any real division.

The problem is I need low humidity so I can glue braces and right now there isn't any practical way to reduce it below about 70%...

I heard of people heating the top in a hot box for a few hours before gluing braces... how does that work?

I don't really want tops potato chipping, but there is no practical way I can ensure low humidity either...

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Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

Well, what are you going to do with the braced top after you put the braces on? Let it live in a high humidity environment for the rest of the construction of the guitar? Focusing on RH control just at the time of bracing, and ignoring the rest of the process, doesn’t make any sense to me. You should figure out a way to tent a small area of your shop and control the RH in that small area with a dehumidifier. The guitar should mostly live in the controlled RH space and come out for some work on it, then go back in when that work is done. It sounds like you want to do the opposite, and I don’t see the logic of that approach.

It's easy to build a little glue up room out of 2x2's and poly sheeting. Low cost, quick to knock together...

Yes, build a space. You can mess with heated cabinets and the like but it’s less complicated (and less risky) to just build a space, big enough for a workbench and whatever tonewood supplies you’ll be using in the near future, but small enough that it’s easy to hold at your target humidity level by running a small dehumidifier.

I built a 9ft x 9ft dry room in my workshop, from cheap and salvaged materials. One dehumidifier can keep it at 40% rh through a sub-tropical wet season. All assembly and a lot of repair work happens there. In this small space I fit a goodly portion of my wood stash, 9ft-wide-workbench-slash-go-bar deck so I can brace multiple tops and backs simultaneously, hangers for 5 assembled instruments and three in the process of being lacquered, plus storage for another 10 or so separate body/necks. Way more space than I need as a one-man operation.

I can’t even imagine trying to build and repair guitars without this dependably controlled environment.