I'm in central texas so our winter's aren't that extreme. It'll be 40s-60s most days with little jaunts into the low 20s on some cold nights and into the high 80s on really warm days. We can grow lots of brassica's, spinach, lettuce, etc all winter pretty much.
I've got a spot that I sheet mulched/lasagna gardened last spring. I put down cardboard, composted horse manure, spoiled hay, some clay-loam dirt. I planted red amaranth into it and it did pretty well. I've harvested the seeds and chopped the amaranth down. I left it where it grew and have since remulched it. I've covered the entire area with a few inches of leaves and then a few inches of spoiled hay.
In another part of my yard I have an area that's pretty heavily mulched with hay as well. Some vetch seeds accidentally landed in the hay and have since sprouted. I'm wondering if I could sow vetch or clover ontop the hay mulch? I think it could work... then when I'm ready to plant in the spring I just chop the vetch/clover down or just chop down sections and plant in between. I'm just concerned whether or not the vetch or clover can get it's roots down through the many inches of mulch to get to consistent water.
Highly doubtful that small seeded anything would grow. Depends on how often you water and how decomposed the mulch is. I would pull back the mulch in rows and plant in soil. Pill bugs will probably eat most seedlings, so plant thick.
I'm planting Bluebonnets (Lupines for the non-Texans here ) as one of my nitrogen-fixers. They have pretty big seeds. What about a large-seeded legume for your nitrogen fixer, like Bluebonnets - or maybe even Fava Beans, which do very well for me here a bit northwest of San Antonio. You'd need to tuck the beans into the mulch somewhat to make sure they are close to the soil and won't dry out. Or you could make a little pile of soil where ever you want to plant a bean.