Stand development will give you even less contrast than what you are experiencing with Rodinal 1:50.
If your negatives are lacking contrast and HP5+ is producing less grain than Tri-X are your HP5+ negatives being underdeveloped?
As has been repeated over and over again on APUG, stand development is a specialized technique used for contrast reduction. It was never intended as a general purpose development method. It is used along with the Zone System for tonal compression. Is there a particular reason you want to do this other than curiousity?