I don't have rear coilovers but the obvious concern is the ability of the top damper mount section to cope with the load placed upon it in this case, rather than through the usual upper spring mount.
I am guessing you would need to reinforce the rear tower? No doubt this has been covered on toymods, I would suggest searching there as I am sure there will be a diffcult to follow answer involving metallurgy and drawings with lots of load calculations.
Amongst all that however will be an informed & reliable response.