The first trick was to find the leaks. The gaskets seemed fine, but throughout the drysuit, most of the sealing under the seam tape was dried and flaking away.
In an early attempt, I sealed the gaskets for bag clips, and then inflated it using a SUP air pump.
World's Funnest Inflatable Humanoid.
I then wrestled it - which must have looked really interesting to any onlookers - compressing first the legs, then the body - listening and pressing my face up in places where I thought air was escaping. The legs seemed fine, but the body was leaking air. However, I couldn't determine exactly where it was escaping from.
I took it home and hung it in the shower, partially filling the drysuit with water and then letting it hang in different positions to see what leaked. This was an improvement, but still inexact. I couldn't see anything dripping from the legs, but clearly water was escaping along the chest zipper. It came down along the relief zipper as well, but not as much. However, it would run down the edge of the fabric and drip like a stalactite, so I wasn't sure where along the zipper it was leaking.
Filled to the ankles.
The next test helped me narrow down the leaks. Following advice from a friend, I smeared liquid soap all over the zippers and their seams. I then inflated it - with deep chest breaths, no pump, because I didn't have one with me - and then squeezed the body much as I had before.
I have to say, by this point my technique in compressing the drysuit to check different areas of its surface was much refined.
Lo and behold, little bubbles of soap appeared, in varying sizes, along the chest.
So, clearly not the zipper itself, but the seams around it.
If you look on the other side, you can see where there's just not much backing up the stitching.
The next step is to see if there is a good way to re-seal that seam. I'm guessing AquaSeal, or something similar, but I want to take time to consider how best to make it a continuous, singular spread. I don't want any gaps in coverage.