I have never heard that there are more enzymes in specific areas of the skin that allow more conversion. I would honestly like to see a source on this, because from my understand - as I stated above - it doesn't matter the route of administration. Yes, you use less in a transdermal because it absorbs better than orally, as with all steroid compounds who's molecular weight is under around 300. But regardless, if you injected it, there is no special injection sites with more enzymes that make a compound convert better. All of that happens in the liver (and sometimes kidneys) and does not happen at the site of administration. This is true for prohormones/prodrugs only. They make their conversion to the parent steroid in the liver, whereas things like testosterone simply slowly release from their depot injection site and go directly to bind to the androgen receptors and do all the other things test does. After that, it passes into 2nd pass metabolism and is processed by the liver.
If you took DHEA, bound an ester to it, suspended it in oil and took it, it would work much better than plain jane DHEA. But that isn't to say oral DHEA or pregnenolone isn't orally bioavailable. Because these compounds are very simple in structure, they can be orally absorbed just fine and pass into the liver where they are converted to their downstream hormones. As a matter of fact, pregnenolone is now being looked at as a cure for depression and is being given to mothers who have post partum depression as an injectable form and is working great. This is because pregnenolone is also a
neurosteroid.
But, that's a whole other subject.