There's been some discussion about in this past. I remember the conclusion being that it seemed ultimately too risky to explore. It is possible to burst or break an erection, afterall it's engorged! There was some talk about causing leakiness in the future which of course would mean soft erections.
There are mechanisms which close off outflow of blood: this is what causes an erection to occur. The inflow continues, but the outflow is restricted. By adding pressure you may injure the outflow restriction ability. Clamping may not harm this because it is occuring before the mechanism and therefore the mechanism doesn't see this additional pressure to weaken it.