It’s now possible to use some keywords in NetRadiant gamepacks to set engine folder that may differ according to the user configuration.
For example it is possible to set paths using %ProgramFiles%, ${HOME}, ${XDG_DATA_HOME} and other things like that in a way the string is rewrote according to the user environment.
Maybe one day this feature will be usable for game user directory too, we’ll see!
Note: this feature may not be implemented in other Radiant so a gamepack making use of this feature is specific to compatible editors like upstream NetRadiant.