: I recall that on my Cavern Megabattle setup I often would grab a Ghost and
: use it to get to the top of the little box out in front of the entrance,
: standing on top for a good Sniper angle. If the Ghost was too close to
: either edge, a Marine would somehow hop into it.

If you pause a hog near the back edge of the Marines plateau in the rockslide area of level 2, and there's a Marine down below, he's potentially able to board despite the rediculous distance. So, that box height will be no trouble at all!

: The Ghost would raise up, with me on it.

I must give that a go. Sounds amusing. Up on that box thingie, his driving AI will be offline I think. An Elite on a Ghost would likewise just hover there (I've had them on the exit side), though he's able to rotate the Ghost at least (and I expect a Marine can too).