What products might be out there to "pour your own"?
As far as my searches could determine, none. All were premade sheets of some sort. Quick and neat, yes, but don't fit into my existing spaces, and are somewhat too pristine. These twisty secondary roads are not well maintained and follow tight turns and non-level ground.
Thinking about products I had on hand, could I DIY?
So far, yes. The base aggregate was some fine sand, blackened with a diluted solution of Fiebings leather dye, which was also added to my diluted white glue to stick it in place.
First, prep the surface. No unwanted bumps or bits of debris. Trim up the edges with a razor or exacto.
I used a bit of rolled Silly Putty to keep the roadway from covering ties and rails.
Next, I added beads of white glue to act as curbs for the edge of my roadway, and hopefully keep the sand in bounds.
Next comes the sand. Poured in with a spoon, and smoothed with a business card. A light touch is required, barely brushing the surface until you're happy.
Next, I poured some dye into a scrap plastic jar and diluted it with isopropyl alcohol to 50%. This was carefully "droppered" on to the sand until it was all black. Go lightly near the edges or it will bleed into the surrounding scenery (mine did).
Add the dye solution to your thinned white glue (like gluing ballast) and cover the "pavement" with glue with a dropper. Like the dye, be careful near the edges.
Now we wait until it has thickened. You can go in and clean up the edges with a razor blade if you like, and use a tool to get any sand/glue out of places you don't want it.
Once it's firm, you can remove the putty and vacuum up anything loose. Looks a bit like fresh laid blacktop (without the stink ;-) Once fully dry, I will drybrush with some gray to weather it, and add stripes. Then I'll have to clean up the edges where the dye bled.