On this subject, from what I gather stills don't just ferment the sugars you put in them like the fermentation we are used to does. You get a much cleaner product than one you would get from racking off - rum comes out unflavoured, the flavour is lifted from the bourbon barrells that it ages in.
Now - please don't respond angrily if that's incorrect as that is ALL I know about it - hahah. My point is that if you were to make something 20%ish, you'd want to take a lot of steps to round off the crap flavours - wine filtering, degassing, ageing, etc. There's a reason the best whisky is aged for decades - it'd need some research to see what would at least be the best introduction to it, nevermind what's the most viable.
I'd be interested in trying something if anybody's up for it, maybe we could organize a small swap to see end results. The idea of having a nice coconut rum or something on ice in the summer is very appealing, and I can't imagine it'd taste too different mixed with coke, you'd just need double the amount - that and having the ability to make low strength "spirits" under your belt is bound to score some brownie points :p