My first intended use is simply to fill extrusions and use the polymer to dampen resonance and to add weight to the extrusions.
I had been contemplating a bridge type mill in the future and depending on how my first one works out and how everyone else's tests and machines come along, it may get designed and built using this stuff. Maybe a 36" bridge.

One real failing on the design of the little X2 that I have is the center of gravity over the cut. It is way over extended out front and with one mounting point, is a weak spot. Most of the accuracy issues stem from that deflection there. The dovetails aren't perfect either and require a lot of fiddleing to keep any type of reasonable accuracy with mild steels and very minimal cuts.
My 80/20 machine is using large linear ways and has a better mounting for the column, but the cutting forces will still be out front.
It should work better than the X2, but I think a bridge type mill will equal out some of those forces and put them in line with the structure of the bridge itself. Less flexing under load there.
The drawback is the table size limit, but if the bridge is large enough, it would cut some decent sized parts.
The base would also be polymer of course.

More on rebar.
Reinforcement doesn't have to be rebar or rebar like.
Rebar works so well because the aggregate actually locks the ribs of the bar in the mix and when pressure is applied to it, the rebar is like in tension and isn't as flexible as it normally would be.
A smooth pipe will provide some added umph, but not as much as rebar because of the ribs.
You could put bends on the ends of the smooth pipe and make them less likely to slip through the pour under tension, but it still won't deliver the same ultimate shear strength as the rebar.
Filling extrusions or steel members are a good substitute to rebar on gantry spans. Think of it as an exoskeleton and rebar as an indoskeleton.
Anchor points will be needed for bolting parts on and when designed well, can be incorporated as part of either type skeleton to add strength.

More on heat.
I mentioned pours less than 4" as thin. I think I read the other thread where he said he doesn't pour thicker than 2". I think that is plenty to mix at once. It would give the opportunity to add the structure if indo during the second or subsequent pour. I think this thickness is more controllable and far less likely to produce any kind of threatening heat.