I couldn't think of any way to do that directly, or even indirectly, entirely within the new traits.
The best I could come up with is to have the original skill, Artist (Woodworking) in this case, grant a bonus equal to its points to the sub-skill.
The one hiccup to that, is that there appears to be a bug in GCA (both 4 and 5) where if I use upto() to limit the sub-skill to 0 points, it doesn't actually allow any skill level at all. So, to implement the bit about not letting them spend points in the sub-skill, you'll have to wait for me to release the bug-fixed version of GCA5. But, until then, you could do something like this:
Code:
Artist (Woodworking), IQ/H,
default(IQ - 6, SK:Carpentry - 3, "SK:Artist (Interior Decorating)" - 4, "SK:Artist (Scene Design)" - 4, SK:Artist - 6),
page(B179), cat(_General, Arts/Entertainment, Craft),
gives(="me::points" pts to "SK:Artist (Woodworking - DX)")
Artist (Woodworking - DX), DX/H, page(B179), cat(_General, Arts/Entertainment, Craft),
page(B179), cat(_General, Arts/Entertainment, Craft),
upto(0 pts), needs("SK:Artist (Woodworking)")
Again, that upto(0 pts) should NOT be included until GCA5 gets a fix for it (and GCA4 will never get that fix)--so not putting points into it would be up to the users to remember for the time being.
That's the best I can come up with right now. But because the sub-skill is based on DX and has no defaults, it'll always be based on DX and only the points granted to it by the main skill.
This is a bit more work than something we could entirely build into the sub-skills, but I can't see how else to do it.