Yes, a UTH is fine as long as you run it on a thermostat. I run mine on a proportional thermostat, and it never gets too hot to burn her (when you set it up, just make sure you feel the glass with your hand). The most important thing is to ALWAYS bury the thermostat's probe into the substrate. It will require a little tweaking to get air temps where you want them when you do it this way, but if you don't bury it, the temps under the substrate could get too hot.
In fact, I'd recommend a UTH as the only source of heat - bright lights tend to disturb BRB's. They are extremely secretive animals. Mine instantly retreats into a dark, cool spot as soon as she's exposed to lights; although I know there is at least one keeper on the BRB forum with the opposite experience.
Definitely go ask around there... here's the link:
http://forums.kingsnake.com/forum.php?catid=189
Nobody seems to be able to find that forum anymore since they changed it to a Boa subforum. 
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::my homepage > http://www.winds.org/nekomi
My Growing Zoo:
1.0 Husband (Byron) ^_^
0.1 black cat (Shade)
0.1 Brazilian Rainbow Boa (Zia)
1.0 Leucistic Ratsnake (Houdini)
1.2 Cockatoo cichlids (A. cacatuoides yellow-gold)
1.2 Panduro cichlids (A. panduro)