Awesome thanks so much for this info @cel .. helpful, also @zelf . I think in the end its probably worth the hassle to be able to switch and change wallets and ssb feeds so perhaps its ok to have to like sign a message on your feed saying this is your public eth address (and vice versa in an eth transaction of some kind ?)
But that said could be handy and fun to be able to derive a 'default' ssb feed from an eth address in a predictable way. I wonder if that woudl be a specfication of some kind - could be a very short one pager - and if so where one would publish such a thing? Perhaps as an library beginning "ssb-xxx" or something?