Sorry you have received any replies, but for the most part we on BE don't know enough to give much advice. Somewhere in the back of my mind is the notion that Scotch is finished in Sherry or Port Casks for around a year? I think what you are doing sounds very interesting. I enjoy vatting bourbons and aiming for something a bit different. I have added cognac to bourbon before but the ultra aged bourbons tend to be slightly like cognac. I have never considered adding an excellent quality rum to bourbon or to scotch, but why not.
I would say take maybe 30 ml of Macallen or Aberlour and add a few drops of rum and play around with that and see what you think. I will do the same, since I have a very good sipping rum and several good Scotch whiskies (most are cask strength, however), but I can try it with a couple of Johnny Walker blends........... which I find to be very good Scotch.
There are not many Scotch drinkers (at least those who acknowledge being such) here, but I applaud what you are doing. Let us know how it turns out. If my results are of any interest, I will follow up.
Do not go gentle into that good night,
Old age should burn and rage at close of day;
Rage, rage against the dying of the light. - Dylan Thomas