Chris,
I have used seacast on two projects. I have been lucky, the first I was able to access the transom from inside, under the splash well (outboard). There wasn't much interior skin on it, and the plywood was toast. There was a section at the top of the transom glass under the outboard that was broken. I cleaned out the old plywood, put in an "inner skin", tabbed it in with glass, then filled it up from the repair area in the top. I was painting it so it didn't matter, and had to be fixed one way or another. The other was an inboard, and I was able to do more or less the same thing, but since it didn't go all the way to the top cap I was able to fill the seacast from the dog house.
As Sphelps mentioned, almost always the deck is also toast if the transom is.
By the way, Seacast is a great product, love it, but there isn't an "easy fix" to transoms. I will never have another wood transom, or stringer for that matter. Once you do it, you don't ever have to do it again. Great product, but there are lots of guys on here who use wood. Devil is always in the details, but if you get it right you will save some money.
I have links in my signature to both builds. The outboard is all pics. I have some videos of the inboard that might be more informative.
Can you access the inside of the transom from the hull? My guess is no from the pic you posted. As Sphleps mentioned I have seen people build tools to get to the hard to reach areas, but you might be better off popping the cap. It usually isn't as hard as it sounds.
