^ Yes, I have grown green_bell-jalapenos. They were exactly as you'd expect, a cross in size, shape, taste and heat level. I was not trying to cross them, I had just reused some seed from a past year's crop and I don't even recall now if I had expected it to be a pure bell or a jalapeno, which fruit the seed came out of. I don't think I have tried to raise seed from that F1 generation so I don't know if it is viable or not but I suspect it would be.
I don't really see much point in the bell jalepeno cross however, there are americanized jalapeno strains that are milder in heat level like the cross is. Then again, some people can't tolerate much capsaicin and might think it's a great outcome.
I can't recall all the unintentional crosses I've ended up with in past years. There's a white-red habanero, jamaican chocolate-red habanero, cayenne-jalapeno, etc. Some others I could only make poor guesses about since they weren't intentional crosses. In most cases I prefer the pure peppers over the crosses except for one case where I had a hybrid that had excellent pepper shelf life, they'd not dry out or rot for multiple times as long as other peppers I've dealt with. Unfortunately I don't know what type of peppers crossed to make it, except that the plant itself looks to be at least half C. chinense. The F1 generation seeds from it are viable, I'm growing a new plant from them this year and I overwintered that plant from last year.