Rowland:
Compost that includes green manure crops will contain substantial nitrogen. Whether the green manure is grass, comfrey, clover, or mustard, it all contains nitrogen. For crops that are used as forage for cattle etc., it is common to measure total nitrogen to measure the protein content of the forage ("crude protein" (CP)). The CP value of forage is just the forage lab's measurement of the nitrogen content in the forage, and multiplying that nitrogen number by 6.25. The amount of nitrogen varies by crop type and condition.
A forage grass crop, managed for maximum yield and quality, harvested at the optimum time, may have 12% CP (as DRY matter, green hay will be less, due to the water content). Divide 12% by 6.25, the dry matter is 2% nitrogen. Compost that hay, some of that nitrogen is available quickly, other becomes available slowly as the compost is broken down by insects, fungi, bacteria, worms, etc. (so prior year compost becomes a slow-release "nitrogen reservoir" in your soil). Old hay beyond it's prime will have less CP and therefore less nitrogen (but still has value for compost!). Other crops may have higher CP content, others less.
I think that if manures become less available to you, green manures (or compost from them) are good choice. I'd wager you already have a good quantity of nitrogen "banked" in your soil, due to past horse manure applications that you have posted about!
Added later: Let's say you have a 50-pound bag of compost, from green manure and dry matter ("browns", leaves and so forth), and it is 1% nitrogen. The area you are covering with that bag is getting a half pound of nitrogen! Think about the area over which you use 50 pounds of compost, that is a LOT of nitrogen!