Matt,

At least three factors argue against just pouring resin;

• Expense. All resins cost more than slabs of cheap plywood or offcuts of solid wood
• Exothermic action. All pourable resins, even casting polyuthane and casting polyester have a limit on thickness that can be poured at once, commonly 10 - 12mm. Exceed that, and the result is fumes, smoke or even fire.
• Weight. Most resins and fillers are more dense than plywood.

But a slurry containing something that will soak up heat will work. I've used sand and polyester resin (to bond in iron ballast ingots), but that adds weight. I'd think in terms of a slurry of resin and glass bubbles, not too stiff, just fluid enough to pour out of a bucket and needing just a little smoothing over to a level surface.

You will still need to build up a socket to house the heel of the mast, and slabs of thick plywood remain the most convenient way to do this. Here again, a pourable resin can be used to advantage, with a tapered mast heel, leaving the socket oversize by 5 - 10mm, coating the mast heel very well with release agent, and pouring resin into the gap after stepping it.