I think it’s reasonable to assume that at least some length gains can be made from the tunica, since ligs couldn’t be a factor in girth gains, which also happen. I’ve never really stretched BTC and managed to gain (not spectacular ones, but gains just the same), and I’m confident that a good portion of that is tunica in my case
Lig gains are considered fairly permanent in the sports injury world. There have been some experiments in trying to shorten them by using heat. In much the same way that there is a heat range for extensibility, there is a higher heat range where proteins can be denatured which causes contraction. Which, while on that point is a good reason not to use extreme or painful heat.