To turn a pdf into a writable editable one, you just have to:
-turn your pdf page into an image (open it in photoshop or GIMP and save as png or jpg...)
-make a new document in open office, import the image made previously as background
-place the fields that can be edited on the page
-export as pdf form
On a simple sheet, you can do it in less than ten minutes.
You can now fill the sheets and use them during play. You can exchange them by using any online storage site.
As I said, I think your solution is probably better than mine in the long run, because it enables to store and use character sheets within Roll20. And, being a character sheets fan, I would probably like to make some sheets, adaptated to online use, the way you describe it. I can even imagine this kind of character sheets made available through the marketplace.
Your solution would make it possible to produce some beautiful sheets if some work is put into it. It just seems a little complicated to me. So, maybe not a solution that could be quickly implemented (but the devs can prove me wrong).
But, I see the capacity to read and save pdf directly in Roll20 as a quicker multifunction feature. At one stroke, it would enable the use of character sheets, exchange of documents and handouts, or even consultation of rulebooks. And you would also have the possibility to use all the already made pdf character sheets floating around the net.