I have been exploring what is available in 15mm for BOB armies/scenarios, outside the obvious ranges covering WWI/RCW. There seem to be several interesting miniatures available, which could be really useful -- but could also be easily missed because listed in different ranges/periods. Some are cast for earlier periods, but I usually operate under the assumption that, at the scale 15mm are designed, differences can be negligible, as long as hats and weapons are a reasonable match (this is particularly true for irregular forces and militias.)
Here is what I came up with. I hope this will help others, too. The list is arranged by manufacturer, range/period, code, and description.
IRREGULAR MINIATURES
Crimea RCW3 Caucasian rifleman (in assorted positions)
Balkan wars SBWB3 Bulgarian infantry advancing in fur hat
Balkan wars SBWS7 (Serbian) Field gun & 4 crew (in fur hat)
Armies of the Caucasus CAU 4 Caucasus cavalry with musket and sabre
Armies of the Caucasus CAU 8 Chechen infantry with musket
Armies of the Caucasus CAU 12 Caucasus infantry with musket
Armies of the Caucasus CAU 17 Georgian Aznauari infantry with musket
PETER PIG
WWI / RCW 75 Dismounted Cossacks
WWI / RCW 96 Russian infantry in greatcoat and furry hat
WWI / RCW 265 Serbian Reservist infantry woolly hat
Heads 65 WW1 Turkish (could be useful for 1919 regular Afghans)
Heads 15 Furry hat, Russian
Heads 62 WW1/2 Cossack heads
Heads 76 WW2 Ushanka Russian furry hat
Heads 72 144th scale pilots for WW1 planes
MINIATURE FIGURINES
Crimea 15K Caucasian Light Infantrymen
Crimea 15KC Caucasian Lancers
Crimea 16KC Cossacks
WWI 3WRC Circassian Cossack
WWI 5WTC Syrian Auxiliary with Rifle
LANCASHIRE
Russo-Turkish War RT/T09 Turkish Dismounted Cavalry
Boxers Rebellion BRC14 Chinese Irregular Cavalry
(Also worth checking: 18mm Ninteenth Century Asia / Tibetans)
QRF
WWI WTI07 Turkish Infantry in keffiyah
There is more to do, especially in regard of the Chinese. From Taiping Rebellion to Boxers Rebellion to the Warlords Age, there are several options to explore for figures that would fit well in a BOB setting. I'll share those at a later time, when I'll find the time to expand the survey.