Thanks to Hendricks266 the UserMaphack system is implemented and working as of EDuke32 r4886 (source r4884).
To get the most out of it until the next HRP version (5.4 I suppose) gets released, I recommend to keep using the UserMapHack Pack in addition to the HRP (consider it an update pack).
The new default (hierarchical) version of this pack requires EDuke32 r4884 or later to work. Although you can try my backported r4495 version from the download page if you should run into trouble.
The default 5.3.565 download has been updated to the current contents.
The download link has been moved to "my" thread.
Info and change log -> Post #1
Download -> "My" thread