"Have you tried the opt key during startup? This allows you to select which volume you want to startup on."
Been there done that. I have owned Macs since '95 and this is the cake!
Um... Ok. I knew that how?
That step wasn't in any of the posts so far. Mentioning cmd-R in your last post - which definitely wouldn't have booted to the external - led me to make the suggestion. I understand your frustration, but "biting the hand that feeds" won't help... Oh,
My Mac experience goes back to 1984*

.
Anyway, about the only thing I can think of is a possible problem with the partition map (assuming the Restore block-copy (with "erase destination... checked) completed normally).
Startup again on the Restore HD. Find Terminal from the tools/utilities menu (don't remember exact name). Use the following command to get a list of the device's partition map details, and post the results. That might give us hint of where to go next.
There's no problem I'm aware of with Lion and FW. I have 5 FW800 drives always connected to my MBP, including bootable partitions of Lion and SL installers, "Emergency" Lion and SL installs, and even the Recovery HD. I also have a FW800 bus powered drive I use for imaging my internal that I just plug into the end of the chain.
*(in addition to being a s/w engineer since the pre-dawn of GUIs)