If you consider delrin, why not polyurethane? they're pretty stiff.
On a little search I did, I found the following post from this thread
(which dates from 2007):
From their site, it seems they didn't go through with the idea. All they have is a rubber isolator kit for aluminum bushings
(which they don't seem to offer either). It might be worth it to contact them to ask questions about why they choose to not offer these bushings (alu or delrin).