I have not... at least so far... There always seems to be at least two trees that are 10-20 ft apart where I've setup camp...
I've seen pictures of people urban camping in parking lots hanging off of cart corals, or between a light post and a dumpster.
That being said, I still fear a time when I will find my self on the open prairie with no place to hang. (If I do ever make it up to the mountains, and rocky terrain I plan to carry some climbing nuts to hang from the rocks)
I know that it only takes two long sticks to turn my tarp into a floor less a-frame tent... and with the motorcycle acting as one support really only one stick, but I can't hang off the motorcycle (at least I don't trust the idea... and the angle I'd need to be at if it did fall over, it'd fall over on top of me...) I can even use the hammock as a ground cover/mosquito netting... but it won't be near as comfortable... everything else is basically the same, stay warm with a sleeping bag/quilt, and sleep on the ground.
But that's still on the ground... and the ground is really uncomfortable.
There are other options to keep you off the ground:
World lightest hammock stand - Handy Hammock
I've not gotten one yet, and I don't believe I can fit it into my luggage (even folded up) but it can get strapped to the bike and I'll be fine pretty much anywhere that I have enough space to set up up the stand...
also when you are building your hammock (straps, or whoopie slings, or whatever you use) you can check out this site:
Hammock Hang Calculator - The Ultimate Hang
To figure out who long your suspension needs to be... I started with how high I could possibly reach to get the straps around a tree, and then worked off of that to see how far apart the trees could be, and that used that distance minus the length of my hammock divided by 2 straps to get the length of each strap... and then added some margin of error. (for when I can reach higher because of how the terrain is laid out)