Glad it's helpful. Yes, the "face layer then machined" method is one of the best I know of, assuming the face layer is hard enough. Agreed with Gerry that MDF is too soft. For rail mounting, we don't want to yield/dent the material, so traditional "hardness" definitions/tests don't apply (those typically involve a permanent penetration of a ball indenter). The Modulus of Elasticity is a pretty good measure for rail mounting purposes, assuming the material is strong enough to not permanently deform (dent). MDF would probably dent while epoxy may not. Here are some options with MoE (units are Mpsi or psi*10^6):
Self Leveling Epoxy: 0.25 Epoxy.com Self Leveling Seamless Epoxy Mortars Epoxy Grouts Zero(0) VOCs
West System Epoxy: 0.45 WEST SYSTEM | Epoxy Resins and Hardeners - Physical Properties
MDF: 0.5
Tempered Hardboard: 0.75 http://www.fpl.fs.fed.us/documnts/pdf1993/mcnat93a.pdf
Devcon steel-filled epoxy: 0.85 http://www.devcon.com/prodfiles/pdfs/fam_tds_101.pdf
Paper-based phenolic (Garolite X): ~1.3 http://www.professionalplastics.com/...DataSheets.pdf
Glass-based phenolic (G-10): ~2.2 (same source)
Epoxacast metal-filled epoxy: 2.0 - 7.2, evidently they use a lot of filler http://www.smooth-on.com/tb/files/EPOXACAST_655_TB.pdf
Aluminum: 10
Steel: 29
A caveat with MDF and hardboard is that those published MoEs are along the face direction; the perpendicular-to-face direction (what we care about for rail mounting) is probably not as stiff. For solid wood (oak, maple...), it's around 10% the stiffness of the long direction, so ebony, lignum vitae, etc, are of little help. This caveat probably applies to the phenolics, although not as drastically.
If using a jointer to level, the best option appears to be paper-based phenolic. I've never jointed it, but it appears possible: http://www.sawmillcreek.org/showthre...d-how&p=785891
For more surface stiffness, another option is to first bond the face layer (e.g., hardboard) and joint/flatten it, then top that with aluminum (or cold-rolled steel) strips to stiffen the area just under the rail contact lines. E.g., two strips of aluminum, 2"x3/8", bonded with very thin epoxy. Aluminum or CRS strip thickness is normally within a few thou tolerance; if clamped tightly and uniformly to squeeze out all but a few thou epoxy, the resulting surface should be pretty flat. As long as the alum is thick enough, the softer hardboard layer will not have much effect on the overall stiffness, and may provide some damping. For alum vs steel strips, I'd match the strip material to that of the gantry tube, so thermal expansion doesn't cause distortion. If using hardboard as the 'sandwich' layer, I'd segment it somehow (e.g., kerfed every few inches) so that its moisture and thermal expansion don't cause it to debond.