I have a very active imagination and don’t know how to keep a 50% erection constantly. If you do, please let us know. Although I am an advocate of using a cock/ball strap for hours at a time after PE, I don’t get 50% constantly, either wrapped or with the strap. I can easily double my normal flaccid hang doing this, but I am definitely a grower, not a shower.
Alternatively (to constant erections) consider this: the more new erections you have, the more you are likely to have; and unused cocks begin to atrophy; that is medical fact.
It is not being hard constantly that likely helps increase size gain, imo. More likely it is the introduction of re-oxygenated blood that promotes healing/cell division after a PE workout. When you are erect, you are not getting a fresh supply of oxygenated blood. It would make far more sense to me if one were to have as many erections as one comfortably can in a day or a week, allowing them to deflate, then reactivating them. New blood entering (repeatedly) might could speed the size gain process just as nocturnal erections 4 - 5 times a night keep us sexually fit, no matter our size.
Interesting question and one that lives up to your user name. :-)