Agreed, I've never seen anything other than concrete for steep slope drainage ditches. But it seems like geotextile fabric would hold up. Two concrete+stone tracks with a center ditch lined with fabric would be low cost.

Here is how not to do it:

Geotextiles need to be covered to work. Running water will excavate them over time. You do not want a ditch in the middle of the road. Put it on the uphill side of the road so it can intercept water coming down the hill and reduce spilloff over the downwards side so there is less erosion there, too.

Geotextiles need to be covered to work

Perhaps a layer of tar to protect it from UV damage? Or perhaps a thin layer of shotcrete.

Perhaps a layer of tar to protect it from UV damage?

Putting a layer of hydrocarbon-releasing material in a drainage area would be problematic, from a "green" standpoint, anyway.

True, although it is already on lots of roofs and every asphalt road.

I see that pavers or "articulated concrete blocks" are also options. Apparently the latter just adds some rebar to hold the large pavers in place. Makes sense, especially in a "two track" case where you would have 4 edges that need to be contained.

To me, pavers look better than poured and given the way they are made, they use 1/2 the concrete for a given strength. They outlast just about anything else and are available with exposed aggregate if you want a stone look.

So most interesting at this point is fabric then stone covering the whole area with two tracks of articulated concrete blocks and shotcrete lining a center or side drainage ditch. Maybe make all the concrete a natural brown color. I think that is the least amount of concrete and the most natural look that would be durable for very light traffic on a steep slope with heavy rains. Plants should eventually grow through much of the stone areas.