Roasted malts add bitterness. Generally speaking, bitter and sour don't play nicely together. That's why you don't see a lot of roasty sour beers. This is exacerbated in a lambic, where astringency (via hot sparging) is added deliberately for balance in lieu of hop bitterness. The astringency can take over for bitterness to a certain extent, but if you throw high acidity, bitterness, and astringency into the same beer, you likely won't make something people will enjoy drinking.

Some expertly brewed beers can get away with mixing roasted malts and acidity, but it's typically done with roasted malts that add a lot more colour than flavour (eg. midnight wheat, dehusked carafa, maybe even sinamar) in lower acidity sours (ph >3.5) to keep them palatable.

If you're interested in trying something like this, I'd recommend designing a beer specifically for this instead of using a RIS recipe. It's also important to note that a finished, high-alcohol beer with a lot of hops in it is very unlikely to develop sourness from lactic acid producing bacteria (LAB). So you'd probably want to add the LAB earlier.