As you're probably aware you can achieve what you want in FileLocator Pro with:
File name: *.txt
Look In: C:\Base;!C:\Base\foo
The Look In field does support expressions, e.g. to search within only ZIP files in the Base folder
Look In: C:\Base\*.zip
However, this only works for file types that are container types, such as ZIP, RAR, PST etc. I'll add your request to the Issue Tracker.