For custom gui it's often data/art/intrface/
Yep, there's even whole section made just for that (as DataFiles.cfg may be overwritten during update) http://fo2238.fodev.net/wiki/FOnline2238.cfg#Section:_DataFiles
I tried it some time ago. But I wasn't able to load the interface via FOnline2238.cfg. Because of that I don't provide a propper zip ronillon. ^^
#data\gui-mods\Classic.Transparent.Interface.V3.zip#data\gui-mods\Pure Evil Interface v13.zip#data\gui-mods\Pure Evil Interface v14[ManyFollowers].zip#data\gui-mods\Pure Evil Interface v14[Suppi v3].zip#data\gui-mods\Ras Interface CLEAR v2.02.zip#data\gui-mods\Ras Interface DESERT v2.01.zip
include Datafiles2238.cfgdata\factioncritters.zipdata\faction.zipdata\faction001.zipdata\faction002.zipdata\faction003.zipinclude DataFilesMy.cfg
[2238]CustomIni=data\default.ini|data\faction.ini[DataFiles]PureEvilInterface=data\Pure Evil Interface v14[Suppi v3].zip