I live in Haiti and would like to help develop a cabbage variety that local people could collect seeds from rather than being dependent on having access to seeds. Cabbage is cash crop here and it would help them very much to be able to save seed. Our climate is subtropical and does not have cold winters. It gets a little chilly (like 10C) but not cold enough for most cabbage vernalize and make flowers. Does anyone know where I could find information about what temperatures different cabbage species need to vernalize? I was also considering that maybe I could try crossing some that actually is willing to seed here with Chinese cabbage. Do any of you know if a cross of the two already exists that I could get seeds from? Other interesting cabbage crosses might be helpful too.
I believe Senposai Greens is a cross between some member of the cabbage family and komatsuna, a member of the Chinese cabbage family. Seeds are widely available, at least in the US. It might be useful as breeding material. It may not suit though because it doesn’t make a head.
If you sow cabbage and just let it go does it flower where you are? If not, it might not be possible to develop a locally adapted cabbage. Why not try developing a locally adapted Chinese cabbage as this species, Brassica rapa, is much better adapted to the sub-tropics.
I’m actually not sure if the cabbage would seed here but I suspect probably wouldn’t because my collards haven’t flowered in the three years I’ve been growing them. I am experimenting with napa cabbage at the moment and may just go that way. The local people eat tons of wild mustard greens so it other B. rapas wouldn’t be too foreign. The trouble with the softer greens though is that they don’t transport as well as a head of cabbage. To sell cabbage here the farmer fill huge sacks and then haul them 10km on their heads to the market. From there it is put on a motorbike and taken the rest of the way to the city. Anything but a firm head would be absolutely trashed by the time it reached the city. Napa cabbage might work though. Also, the demand for cabbage comes from their common condiment “piklis” which is like spicy sauerkraut with some shredded carrot. I think the napa cabbage would still work. Maybe I’ll start some napa cabbage experiments while I think a little more about a bigger cabbage project.
I was going to suggest crossing tatsoi with other b. Rapa, as I and others have had success creating greens that are quite good in summer temperatures that way. But I don’t think that would stand up to transport, nor would it make recognizable pikils. Napa cabbage seems like a good fit, and we know it makes good kimchi.
You must be right on the cusp of temperature for collards to bolt. All of mine did (4 varieties), with a season low temperature of 6.5 C the last week of February.
You might keep an eye out for Brazilian genetics - I know the university in Piracicaba has developed broccoli, and the internet says their typical winter low temperature is 12 C.
Does broccoli flower in your ecosystem? Broccoli and cabbage are the same species, therefore a cross between them might be selected to make a head, and also go to flower without cold temperatures.
Also, could you pull the cabbage, refrigerate for a couple weeks, and then replant for growing seed?
Or could you dump a bag of ice on a cabbage plant, and trick it into thinking it had been through a winter? One night of cold temperatures is enough to trigger carrots to flower.
I don’t know about the broccoli. I haven’t tried it here. We’ve been trying rehabilitate the saddest soil you’ve ever seen and are only three years in. We’re making headway but this is the first year you could say that we’ve had much of a respectable garden. We don’t have refrigeration at all but I don’t think that would help much anyway because the refrigeration or ice trick would then have to be used every year and would just not be feasible for our neighbours. I guess I was hoping to find some cabbage varieties to work with that are less picky about overwintering temperatures. It would be great if someone somewhere knows what vernalization temperatures are needed for different varieties. Maybe I should just settle with napa cabbage. 
I probably am right on the edge. It sure seems like it’s cold here in the winter. That’s interesting about the research in Brazil. I’ll see what I can find.