OK, I got my amsteel in the mail today. Amazing stuff. I was able to assemble a pair of whoopie slings ~6' quite easily (having never spliced rope before) using a doubled-over length of bailing wire. I figure I'll give these a try, as I can always pull it all back out to into two 12' segments. Making a continuous loop splice was a bit more fiddly... To summarize, the suspension I ended up with comprises two 6' sections of poly webbing, two wooden toggles, two 6' whoopie slings and a whoopie sling adjustable ridgeline (very similar to what's for sale at whoopieslings.com).
A few things learned:
1) The dye on the black amsteel doesn't stay on the rope very long (most of it ended up on my hands). Is that true of all the colors?
2) I should have splurged and bought more rope, as I've already pretty much used up the 50' I bought in making the suspension for one hammock.
3) There is really no difference between a UCR and a Whoopie Sling--the name depends on which ends you connect to the load. This means that there's no need to worry about not being able to span more than 6' with a 6' whoopie... All you have to do is flip it over and live with having to tie a backup knot on loose end of the contrictor. Perhaps everyone knows this already, but it wasn't apparent to me from reading the forums...
Thanks again for all the suggestions!