So, let's say I require a macroscopic pile of Potassium-40 to do strange and wonderful beta-particle related shenanigans with. What exactly would I have to do to a banana(s) in order to retrieve this isotope?

well, the first stage would be to extract the potassium(all of it) from the bananas(limiting yourself to bananas seems a really crap way of doing this, why not just but a big lump of potassium).

 

once extracted, you'll need to vapourise the potassium and centrifuge it to seperate the isotopes(and this'll only work slightly) and get enriched potassium-40

 

to be honest, there are better, cheaper and easier methods of getting beta-emitters.

First you would have to extract the potassium (hint: just buy some potassium or potassium salt). Then you would have to do essentially the same thing as is done to separate uranium 235 from uranium 238. A simple process, but not easy.

So, no need for tropical fruit then? Perhaps KOH will work?

seeing as it contains a much higher fraction of potassium, yes. but you'll still need to get rid of the OH. KCl or K2O would be much safer to work with than highly caustic KOH