okay, got this one figured out with a little bit more research, actually on this site. If you find yourself in the same predicament that I did, you can actually break apart the impeller with chisels. I felt a little bad using my decent woodworking chisels to do it (I couldn't find any of my others - perhaps time to reorganize the shop?), and I will have to resharpen my 1/4"er, but it works. Took a few whacks with the mallet and chisel and I was soon pulling the remains of the impeller out.
Because of the rust that had locked my impeller to the drive shaft, I spent some time sanding and cleaning the shaft. Not a machinist quality of clean, but it went back together with some gentle persuasion, and liberal application of white lithium grease. It's running a test load now, and NO LEAKS!
Pretty impressed that an 18-19 year old dish washer still has easy parts availability, and is still running. total parts? (including a fresh water supply line) $43.32