ah yes, well vertical milling is sometimes refered to as mounting the complete machine vertical.
I have my mill on a slab of mdf, resting but not mounted to my table. I’m still on the fence about cutting out a piece of mdf at the front so I can slide the hole mill over the edge of the table and do the vertical thing. I have not encoutered the need to do so, so I have not implemented that option yet…
Your solution surely is novel and I always like out of the box thinking. You might wanna try and mount some dead weight at the position you envision your extended spindle to go, just to see if the machine can handle that extra weight that far out. it might put a fair amount of strain on the various components that may cause extra wear.
keeping a weight tight to your body is way easier than holding it at arms length. Maybe opt for a small palm router as your vertical mill unit?
Someone had another novel idea on mounting the complete spindle vertically to be able to handle longer stock. maybe look into that idea aswell?