I think in the long run, it is simpler editing the configuration files of the payloads than including complex fixes for permitting using payloads for other add-ons.
Next we would need to change the orientation of the payload attachment point coordinate system for permitting other payloads. Then we would need to use a special logic for the RMS to permit using payloads of other add-ons. Or resize the payload bay to allow integrating payloads for add-ons that had historically a wrong payload bay dimension.
Maybe it is better to adapt a "mer san mer" (Bavarian: we are we) attitude and require payloads to be explicitly adapted to SSU, then permitting badly converted payloads to be integrated into SSU with brute force. What do we have to loose? I think we have a higher risk in trying to please any add-on to be a payload for us, then having a slightly higher standard for the payloads to be transported.
Or in different words: I don't want to spend time for perfectly having the DPS screens of the real Shuttle implemented, only to then transport sloppy made add-ons for other Shuttles or Gliders with it.
I'd prefer being a bit elitist.