Okay, so I've been bombarded with tons of items and unittypes in my mods, so I was going over ways to try and keep it organized while I'm working and doing massive item work.
Here's what I came up with, and I tested it and it worked so that's why I'm sharing it!
For my mod, there are alot of custom unittypes. For example, my mod is 'Limit Break Mod', so I made a Unittype that was named 'LIMIT' [**It had QUESTITEM inheritance so it wouldn't interfere or break anything] and added '<STRING>CHILD:LIMIT' on the base of all my of my custom unittypes, so that while I'm working I can have them all in one place.
Depending on whether or not it becomes an issue I will be keeping this in my mods to help keep it neat.
The only real problem I've found with this method is when you're making custom items of existing unittypes, I haven't found a way to get them to show up under LIMIT without attaching a custom UT that has the LIMIT string.
Anyways, I hope someone finds this useful! It's not a complete method but it definitely helped me
Since it's all xml and directory structure, it wouldn't be too hard to write a winforms program to copy items and add the child attribute.
The interface could get hairy since there are a metric ton of items. You would want to have some way of organizing them. Maybe multiple ways.
Does TL2 still use the GUID for each individual item?
If you don't care for windows, it could also be done as a shell program with perl or python.