Here is my $0.02, for what it is worth:
1. Remember it is a foreign export stout first and not a spiced mojito. If you plan on adding some simple sugars like D2, treacle or blackstrap molasses, I would not go above 10% of the grist. Otherwise, it may get to boozy. I would also replace the portion of the base malt contribution with that percentage of simple sugar to keep your gravity fairly similar. Also, add the simple sugars at high krausen (about 2 days into ferment).
2. If you are using simple sugars, you may want to raise the mash temp a few degrees to maintain some of the dextrins in the beer or use a lower attenuating yeast like WLP002.
3. I would age the beer on oak first then as mentioned serveral times above add a spiced rum to taste at bottling.
4. I would forgo the additional spice tincture. I think that will be too much and will hurt the balance of the beer. Use a spiced rum like Captain Morgan or Sailor Jerry's to achieve a subtle spice character. You want to be able to faintly detect the spice but not have the spice jump out of the glass and kick you in the junk
Also, the oak can contribute some of those coconut, cinnamon or vanilla characteristics you may be looking for. Plus, you can always add the tincture later, if you choose too.
5. Lastly, depending how brave you are...I would dose the beer with Brett for that little authentic old world twang at the end of fermentation or at bottling. Probably Brett. clausenni (WLP645), which is rumored to be in Guiness and old world wood casks.
This is also similar to how Orval packages their beer.
Remeber, balance is the key. Everything should be in harmony and it should taste like beer first, not an oaky, spice soup. So, use a light hand to start with. You can always add more sugar, spice, rum, or oak. You can't subtract! Let us know how it turns out! Good luck!
