I was wondering if while you were driving you ran out of gas would the FEH continue to operate in EV mode while there was still electrical energy stored in the battery? Or, say you go to start the FEH, and for whatever reason, the ICE does not fire up. could you operate in EV to at least move somewhere else?

Check out this thread
Pretty much answers your question.
If you did run the HV battery down to zero you can use the jump start feature of the car to rejuvinate the HV enough to start the ICE.

go and highlight my name and read all posts by clubford00 then click on "running out of gas" i wrote a few months ago it might help. I would have put the link here but i dont know how lol sorry. Dean

And that link is here

If you run out of gas:

The "check engine light" comes on....
The car beeps at you....
The message center says "Stop Safely Now"...
You can continute to coast if you wish...
The car enters "Limited Operation Condition" informally known as "limp home mode".

Limp Home Mode, when you run out of gas, allows you to drive on battery only for as long as it lasts ( 1.5 to 2 miles ) but at a computer restricted speed of 22 MPH.
Why they picked 22 MPH is beyond me, but that's how it works.

I did it once, and the car ran down like a toy.... 10mph, 9, 8, 7, 6....... dead stop.

Oh... don't turn the key off. If you turn the key off, you CAN NOT drive on battery only. Once it's off, it's off till you re-fill with gas.

-John