I can see that working well - nice idea!
DivisMal: I think they'd be OK with 15s, as long as you don't mind a bit of abstraction or just gigantic masonry. When a block's on its side, a 15mm Alternative Armies orc can (just) see over the top of it.
I decided today that my Ral Partha orcs, which have been kicking around for ages (decades, in most cases) would be worth experimenting on with wash-painting. I was planning to make a HotT army out of them last year, but they'd just be duplicating the GW orcs at a fraction of their size. And I don't have enough of them to make a horde-based army, which would require at least 60 for 24 APs. On top of that, they're actually great figures for RPGs, because there are various distinct leaders and characters, and a good mix of sizes and weaponry.
They're also quite hard to paint conventionally. They're very small and very detailed, with much more definition on musculature and faces then their Citadel equivalents from the same period. It's easy to swamp that detail with paint - filling gaps between teeth or clogging the veins (!) that show on some of the limbs.
So, I thought less might be more, and decided just to use washes on them (apart from an initial coat of steel on the metal parts). Given all the detail that Tom Meier and Sandra Garrity have lavished on them, I wanted to see if they'd "paint themselves".
I'm quite pleased with how they've turned out. The only difference in the process from the GW plastics was that these had been preshaded with Agrax and then drybrushed white, and so had a bit more depth to begin with. I don't think it really made any difference, except that the high points may have been somewhat brighter white.
The rest are going to be done the same way. The process was incredibly quick, because these figures don't have much in the way of accoutrements.