You might be able to just puree the bananas, add ascorbic acid or something to prevent the fruit from darkening, pour it into sterilized jars and process it in a boiling water bath to seal the jars. I've never tried this, but it seems like it ought to work. The thread "canning peaches - splenda
" has some good info on canning fruit without sugar (scroll down to the bottom of the thread).
I make apricot jam with a very small amount of sugar--1/4 cup to 24 oz of apricots and 1 tbsp lemon juice (just those three ingredients). I don't add pectin because then I would have to add a lot more sugar. If the banana butter recipes you are looking at include pectin, that's why they require so much sugar. Commercial pectin is bitter and a lot of sugar is needed to counteract the bitterness, even if the fruit is very sweet to start with. But you don't really need to add commercial pectin to jam; you just need to cook it a little longer without the pectin. You just simmer it until it is a jamlike consistency. I would imagine you could do the same thing with the bananas. You probably wouldn't have to cook the bananas very long because I don't think they'd have nearly the amount of water as the apricots do. You might even need to add moisture--water or fruit juice. You could also add in a tbsp or so of rum for a little extra flavor (I once purchased a banana pineapple rum jam that was awesome, especially on PB&J sandwiches).
I will warn you though that although the thread I linked to above says sugar isn't necessary as a preservative, I think that just applies while the jars are still sealed. My low-sugar apricot jam lasts just as long as anything else while sealed, but once I open a jar, it goes bad significantly faster than my other homemade jellies and jams that have higher sugar content. So maybe put the banana puree in small jars (e.g., half pints) and, once you open a jar, freeze it if you aren't going to use it quickly.