Mr. Koch is right about dilution and contrast. My experience is HP-5 initially looks sharper as the grain is more prominent than Tri-X. Even enlarging D-76 developed -135 negs to 8x enlargement factor HP-5 grain intrudes if using a condenser enlarger. So type film is a factor.I was under the impression that stand developing might yield some better results in regards to tightening up some grain--which is actually my primary concern, moreso than contrast. Let me clarify by saying that there is MUCH more grain in my HP5+ negatives than my Tri-X.
But your question is how to reduce Rodinal enhanced HP-5 grain with the end use a scan. Avoid overdevelopment/overexposure. Develop at 1:50 ratio and shoot near the -135 film box speed. Use a tripod vs increasing the EI. If you want a Ralph Gibson look someone in APUG may post his technique and type materials.