James why not go for a half way house.A movement tray with circular cutouts for the crew and the horse.The limber and gun would rest on the surface (you could do two little ruts to act as chock's for the gun)it would fit in with the infantry trays,and removed when the gun was being fired.nothings glued to it .So it shouldn't limit its use, and would help save your paint work.
very emotive use of colour,there James.The white back ground is whats drawing your eye,to the contrasting limber colours not your brush work.as theres no base between the model and the white .
Mark