Many thing remain unchanged regarding model-editing from 4.x.x -> 5.x.x. There are, however, already discovered several changes in the files. Here's some of the recent research on how the terrain files are built in 5.x.x:
MoP - WoWDev
To sum it up, it basically says that there's now 5 parts of an .adt instead of 4 (there were 4 in 4.x.x, and one in 3.x.x). A converter has already been invented by feangren (you might know him as spoutnik), and it can be downloaded
here.
Regarding the .wdt file: Apparently, some changes have been made, but the old format does in my experience still work in 5.x.x
I've had some troubles with loading different M2's and wmo's - I'll have to research this. But to answer your question:
Yes, (probably) everything that was possible in 4.x.x is possible in 5.x.x - I guess that's why new guides haven't been made.