Another thought came to mind...some self-leveling cements have the property that they are thixotropic and as a result, can be placed to create a ramp or slope. Some of them can be applied over a wooden subfloor to a feather edge. While not inexpensive like deckmud, if you were going to go with a surface applied membrane, in theory, you could build a shower pan with the stuff. You would NOT want to use it normally, or if you were going to create a traditional shower pan since it would not be porous, is harder to work, and much more expensive, but for a special circumstance, it may work. I know that Mapei makes at least one product that has this property, and probably others. I'd call them to discuss it if it sounds interesting.
FWIW, SLC products don't really self-level all on their own, but those that exhibit thixotropic properties, don't at all! They all need at least a little help to become level, and thixotropic versions don't really at all unless you manually make them that way. IOW, the stuff essentially stays where you put it, but will 'flow' when agitated (the abbreviated meaning of thixotropic).
Many of the 'pros' here scorn taking time to go to one of the manufacturer's (mostly free) workshops because it takes time away from 'productive' work. I have the luxury of being retired, and can go to absorb that knowledge so I can pass it on. There is a place for both types of learning, but the 'pros' here get really uptight when I point out that what they are doing does not meet industry standards or abide by the manufacturer's instructions. While what they do can, and often does work in their special circumstances, it is poorly positioned for the general knowledge people reading a forum may take out of it...IOW, it may work, and work fine for a special circumstance, but general forums, like this one, are better suited to industry guidelines and manufacturer's tested procedures, not one-offs. What they tend to discuss would be better served on a one-to-one basis, and dispense good, reliable info that won't be confused with a special circumstance on the general forum. They don't see it that way. As a result, we often tend to disagree.
It annoys me when someone promotes a product to be used in an untested and uncertified way, or misrepresents what it can or was designed to do. You can use a pair of pliers to tighten a bolt, but it often isn't the best tool for the job. To some, that is the only tool they carry. There's more than one way to skin this cat that will work.