You can use it. The work is in preparing sensible army lists. The rules for irregulars are there, all you need t decide are
1) What does the average unit represent? (a battalion, 200-300 men?). Black Powder has units in the following sizes: tiny, small, average, large, extra large. Thus you can easily represent small bands, and big battalions using the same rules.
2) Base stats for units and type (Regular vs. Irregulars, formations regulars can take).
3) Any special rules you want to use.
The great thing is the you can personalize units a lot with this system. Thus you can have tiny and small regular units, average sized Greek irregular units, average size Egyptian battalions etc. You can differentiate them by weapons, or skills (ferocious charge, tough fighters, untested etc).
But it does mean you need to do some preparatory work.
Personally, researching the Russo-Ottoman war of 1877 and working on the army lists was part of the hobby and a lot of fun.
goes beyond the straight line.
