There are also a substantial number of people (read: anyone with a natural latex allergy) who could have an allergic reaction to bananas...

the main point of the banana is that for whatever reason it suspends the oat and seed particulates in the solution so they don't all sink. You might get the same effect if you use sunflower butter instead of seeds. Oh and it's also really hard to add marmite without something to smear it on. You can replace the potassium from the banana with more KCl but at some point it starts to affect taste.

Edit: you can also try prunes for the potassium content, but I have no idea how well that will work. They keep very very well, to the extent that powdered prunes are sprinkled on other foods to make them last longer.