Found a really cool feature in Notepad++!
1) Browse to %APPDATA%/notepad++
2) Open functionList.xml
3) In the AssociationMap section, add this to the bottom:
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<association userDefinedLangName="ACS (LT)" id="c_cpp_function"/>
<association userDefinedLangName="ACS (DK)" id="c_cpp_function"/>
Save and close, and restart Notepad++. Go to View -> Function List. Enjoy a functional function list!

The parser definition is kind of alien language to me right now so I don't really know how to specifically customize it for ACS (for example, letting it detect structs and enums and all that stuff for gdcc). And I imagine it would be madness to make parsing rules for DECORATE but, well, using the C/C++ parser for ACS is a start!
Oh and, in case it wasn't obvious: this only works if you're using BlueShadow's custom language definitions because we are associating his configs specifically in the AssociationMap