Are you saying ALLOW_MISSING ~*.ITM~ wouldn't work?
How about ALLOW_MISSING and then listing all the .ITM files I modify? Ice points out I'll have had to have listed them at some point anyhow, though I guess it would add a lot of lines to the .tp2.
Basically I'm looking for the method of least effort.
So, suppose someone were making a hypothetical IWD tweak pack. Cam coded a component for Tutu Tweaks and G3 Tweaks (coincidentally, I have heard G3 is a pretty good place to look for mods) where he patches armour files depending on the type of armour. So he went and found all the armour .ITMs in SoA, ToB and so on and listed them, doing checks for ToB installation so it didn't spurt errors if someone didn't have ToB installed.
If someone wanted to do a similar thing for IWD1 they might note that HoW and TotL add some new armours. So, rather than bothering with ACTION_IF FILE_EXISTS malarkey to check for the expansions, is there any reason why an ALLOW_MISSING ~*.ITM~ wouldn't work at the start of the component?