I want to put new tile over my existing tile in a standup shower. I have owned this home for 20 yrs and I know the waterproofing and pan are not leaking. I do plan on installing new waterproofing over the tile by means of a properly adhered fluid applied material or sheet good waterproofing like Bituthene. Sloping to the drain isn't an issue and I plan on epoxy setting a new drain inside the old brass one. Does this sound acceptable?
Not a good plan. In a typical, conventional, shower, there are weep holes under the surface to allow any moisture that does make it below to weep out and down the drain. Installing a new drain like that just wouldn't work well, even if you could find something to fit. Also, bituthene is somewhat flexible, and not designed for thinset to adhere to, so the combination of the two is problematic.
Depending on the method of construction (hotmopped is common in CA area), while it may not be leaking now, that doesn't last forever. A tearout of the pan is a better plan.
IF you can raise the existing drain (it's often threaded but after 20-years and the mortar and thinset it may not move), and you cleaned things off well, if you have confidence in the longevity of the pan, you could tile over it. Some nice porcelains are barely 1/8-inch thick (I used one like this for the floor of my mother's shower), and you may be able to tweak the drain cover to accommodate it. It's rarely a good idea to patch over an existing shower if you want it to last another 20-years.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013