My first thought is “nah this won’t work”, and my second thought is “well…what if you installed strong springs on each of the Y rails to hold the gantry, compensating for the gantry weight, kinda like the stock Z axis works?”, that would be a wild experiment, but if this forum has taught me anything, it’s to expect to be amazed (remember that very first post from @Vince.Fab that is still alive 748 posts later, and changed the game of aluminium milling on a Shapeoko?)
As long as you understand the forces at play and are willing to acknowledge/accomodate the intrinsic (physical) limits, then many things are possible I guess. It looks to me like you have an opportunity to become 'that guy who engraves vertical concrete walls with a Shapeoko" 
Now, pep talk aside, I would be mainly concerned with how the X axis v-wheels behave when working in that orientation, with lateral force on them (assuming you compensate for gantry weight with springs or something, they would still not really “ride” the v-rails anymore). I guess this could ending meaning the need to upgrade to linear rails, maybe installing them on the (new) top of the X rail.
I’ll be following that thread with a lot of interest