Wow - that's a real pain, Dan. I'm surprised the Ilford hi-gloss didn't work with your inks.
You asked for ideas, so here goes.
Buy a Canon Pixma Pro-100 from Adorama. After rebate, it will cost you $99.00. You get a free box of 50 sheets paper so that knocks the cost down even further. Here's the link: http://www.adorama.com/ICAPRO100.html
I have bought two of these printers and I got the rebate back in the form of an American Express card with $300.00 on it which I have already used up. The rebate came back in a few weeks. I'm applying for the second rebate now.
I've printed on the Ilford gloss and it is definitely tough, tough, tough.
I'm thinking there is a difference between the Epson dye inks and the Canon inks, if the print is not durable coming from the Epson.
I have actually used a little water on a rag to clean up a smudge that got on the print near the border and I rubbed quite hard and it came off and did not damage the print that was printed on Canon 13 x 19 Pro Semi-Gloss. Talk about tough.
The idea may sound far-fetched, Dan, to buy a printer just to experiment with, but if you do the math, with the special they've got going right now, you could try the system, try using pictorio or Canon's gloss, or the Ilford Gloss or all three and after the job be waaaaaay ahead if it works, and out only time and a little money if it doesn't.
At this point, it sounds like you don't have much to lose, and figuring the math on this, you won't lose much, and may gain a firm solution.
Mark at Ardenburg mentioned in a post that he got one of the Pixma Pro-100's and is doing longevity experiments, and said he is definitely impressed with it.
So there's my idea. I had hoped the Ilford would work for you, but I guess the Epson system didn't like it. Sorry about that.
-Mark