If the cedar is well aged it might work okay but it would be far from my first choice and pretty close to my last. In fact for the most part I avoid solid wood for the carcass of a cage due to issues with expansion and contraction which can happen when exposed to heat and humidity differentials. And cedar is one of the worst woods for this. Most of the softer woods are.
I limit the use of solid wood to the face frame of a cage and I would use properly aged and sealed cedar for this.
I have seen cages built from solid wood where at certain times of the year the door was tight and at others it was loose. Bad choice for a live-bearing venomous snake.
Some will tell you that cedar is toxic to reptiles and there is some truth to this, but the danger really only comes from dust and or shavings, which greatly increase the surface area. I think the biggest issue you might have, other than expansion and contraction, is if the wood still has some moisture and the toxic resin migrates out through the sealer. And most cedar sold is quite moist so this may very well be an issue.
At any rate, if you really want to use the cedar then make sure it is properly aged and then I would only use it for a face frame or trim on the cage.
Regarding sealers, I think I would probably use epoxy resin or a clear 2-part urethane or epoxy paint, just to make sure the wood was well sealed. It would have to be a flexible finish due to the nature of cedar. Can't say I know for sure of any finish that is flexible enough and that I would trust to seal in the toxins in cedar.
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Currently keeping:
6.10 Gonyosoma oxycephala (Javan, mixed colors)
1.1 Philodryas baroni
1.0 Rhodesian Ridgeback
