So I played around taking some leisurely drives and sure enough some good mileage can be had with the following stipulations:
-No other traffic around
-You've got lots of time on your hands
The basics of my methodology were:
-have the cruise set to around 25mph or less
-downhills where I can use the speed shift to N
-downhills with sharp turns or stops at the bottom go for L
-D/eco cruise at 24mph is pretty much for uphills and things mostly flat-ish.
Oh, also I've found using Google Maps and choosing the bike route is about the most eco friendly route. It seems to aim for distance and elevation. it will also show you the elevation.
So I did a trail race this weekend 24 miles away. Looks like it's around 1200' of uphill(and down) both ways on the route I took.
On the way over, I did find one spot near the beginning where apparently I did clear 100% and did have the ICE kick in for engine performance. I let it finish off it's un-needed routine of warming up a bit. Other than that I managed to scrape in to the start line with about half the hybrid battery left and not actually starting up the ICE.
I guess it did consider 3 of those miles as being on gas and I only got 99MPGe. Oh... I forgot I did actually keep the cruise more like 45mph on that way...
On the return trip I was entirely in hybrid mode and I think there were only maybe 3 or 4 miles where a van came up behind me and I accelerated to 40 or 55mph a bit. Otherwise I was SLOW. It says I did 16.7 of 23.4 miles on EV and got 58MPGe. Now that's impressive. Though for that matter I never actually put that much effort into my Prius to see what it would do...
It's a shame the shortest route to/from work for me is over dirt and seasonal roads. I literally run the rear wiper with no washer fluid and the dirt just wipes off in dust clouds. But between the shorter distance(meaning less time on gas on the return trip) and this laxidaisical driving I managed 167mpge on the way in and 70mpge on the way home.