Blauk, I've read your post at least 10 times and either my brain isn't working or I may be totally misunderstanding what you're saying but...
Look closely at the ground transitions they are the exactly same as the water transitions depending on which texture is used. Hence the names plb0001 through plb0006.
Transitions are currently based like this (the upper right is terr000x)...
This is the plb to match terr0006...
To base the transitions on the sub-type (010x, 020x, 030x) HT would in effect be giving us 24 textures to work with and we would need 24 plb transition files to match. And I think that would cause a huge rendering load with all of the transitions that would need to take place.
There are ways around the transitions and creating different effects but they are time consuming and require detailed planning of the textures.