Indeed it could work, tape and aquaseal the seams and just add some gaskets. You would probably be best to just buy large Zpacks jacket (without hood or zipper) and pants (come to ~220 grams) then add some latex or silicone gaskets to the neck, cuffs, waists and ankles (probably 300 or so grams?).
As for how to add the gaskets, you could try and aqua seal them directly onto the fabric? Seam sealer sticks fairly well to my zpacks jacket so I assume aqua seal would also work well. You could also try and stitch it on for good measure.
As for if it would be practical... I reckon if you weren't planning to hit the bottom often then yes (as long as the hydrostatic head is high enough?) and it would be a very very light suit. My jacket has held up really well (only 1 little hole in about 50 days of use) and is fairly breathable in cold conditions.
Availability of gaskets:
An example of how to add the gaskets:
Let us know if you are going to try and build it! I was thinking of repurposing a Rab event jacket in the same kind of way. I might have to chop off the hood and remove most of the zipper though which makes me a little wary!!