You might try a product called Mirror-coat sold by woodcraft. I forget who makes it, possibly West Systems or System 3. It is similar to envirotex in that it was designed as a one coat system for bar tops. It becomes very viscous when it starts to chemically react with the hardner. You can feel this happening if you mix it in a dixie cup because it will get very warm. I've had it actually run through the end grain of porous woods. I think if you let it warm up real good, pour it in the goblet, rotate it around so it gets on all surfaces and then pour it out, it would coat the surfaces very well. You could probably wipe away excess from the lip with a rag dipped in acetone. I would let it dry upside down.
This is purely a guess based on using this product for other purposes. I have to make some goblet in the next month or so, I'll try this out and see how it works.