So I’m just a newb here but I’m in zone 5b too and I just planted hairy vetch (like last week) on my new hugel bed to stabilize the soil and overwinter it, and it’s starting to come up now, so probably not too late?!
I am in 7b so it over winters here. If conditions do allow it to overwinter it will produce a lot of biomass in the early spring. I use it to improve my fields. I cut it with a scythe when it has set seed but the seed pods have not opened yet then spread it out where the grass is not growing well and it seeds itself to start growing when the rain returns. Seed can be saved by drying it in a tarp to collect the seed.
Our average first frost here is mid September as well (we think - in a microclimate and still figuring it out), and the wild vetch is currently going to seed. I'd say it's a perfect time to plant.
I like rye as a cover crop and food crop. I don't plant it until we get precipitation in the fall, so that could be anywhere from mid September to mid November. It usually gets planted well after first frost. As long as you get a few days of weather warm enough to germinate the seeds, it'll be good.
Willie Smits: Village Based Permaculture Approaches in Indonesia (video)