I've been fiddling around with the UCW era to try and get the custom/randomized units into new maps, so far I've had no luck.
So, is it at all possible to get custom maps to use new eras?

Yes of cause it is. You can see it at DT or BFX era that has been added to some custom maps. But in that case the mapper added the mod to their map. The other way it is possible, too. I did it for the SWBF1 maps for my RC mod. But you'd need the lua source files. Maybe it will be possible to get the names you need (cp names for example) via hex editor.