Here's the thing: grains have very large sugar molecules that yeast cannot ferment unless those sugars are rendered far more simple by the action of enzymes. Typically, brewers use the malting process (a process where the seeds (AKA grains) are encouraged to send out tiny roots which then forces the complex sugars to break down into energy sources the roots (and yeast) can use. The problem is that this makes what brewers and most other mortals call beer or lager or ale. Not wine. ABV is about 3-6%. Of course, Fruit, the OP, has I think two options. Option one, Fruit can simply take unmalted grains heated to the same temperatures as the OP suggests he likes (Stout or Porter) and uses the grains as the flavor source for the wine, but then simply ferments ON the grains but does not try to ferment the grains themselves. Option two, might be to brew a batch of stout or porter but to add enough of the extract (DME or LME) or grains to double or treble the ABV so that the potential ABV is about 12-14% (and the SG is going to be about 1.110 - 120 (with the assumption that about 15 points will be unfermentable sugars) - Fruit could simply add more , simple sugars rather than barley malt, but that would dilute the flavors and aroma of the stout.
Bottom line, it is as easy to ferment ON grains as it is to ferment on flowers (hibiscus, dandelion, lavendar etc) to make a wine. The challenge here is to make a wine that resonates with a Porter. My guess would be to basically make a "barley wine" by upping the ABV of the beer to 12-13% while keeping the flavor profile of the Porter or stout intact.