[Bruno]

Attribute N/A definitely feels like a metatrait to me. This approach would make it not exactly a houserule per çe, and would definitely make the GMs assumptions about what it implies transparent.

One of the reasons why I'm not comfortable with saying things like "Well if you're Sessile (Portable) and have No Manipulators your DX score is irrelevant is that some GURPS rules do float around attributes, and a not insignificant number of "purely mental" abilities end up using DX to one extent or another (Telekinesis is particularly blatant but Innate Attack is DX based, whether they're eyeball lasers, pure beams of force from your forehead, or exploding playing cards).

This may not be relevant to a particular character, but it is relevant to the system as a whole. I prefer to work from the general case to the specific exceptions, rather than the other way around.

Sometimes One Thing, Sometimes Another suggests that they aren't synonymous; varying impact suggests that there should be varying point costs; the two combined suggests we need a separate module to cover the second idea. At which point a meta-trait seems simplest.