So not only are there Lovecraftian races, science fantasy PC classes, etc. but much of the setting material for BCK fits the Cha'alt by Venger Satanis standard it incorporates it quite nicely. In fact given the material some of Cha'alt might be prime real estate that our PC's might want to take over! There are lots of systems in Cha'alt that can be used to enhance BCK in a workable way for PC's. But if I was going to do exactly this I'd love to incorporate another VS creation I'm talking about Gamma Turquoise: Santa Fe Starport. Drop the Alpha Blue Rpg rules for space craft & then use your own from ACK's Barbarian Conquerors of Kanahu™ (BCK)
Cha'alt has lots of things going for it but the megadungeon really could be used to enhance an existing OSR setting. Gamma Turquoise: Santa Fe Starport gives a place for the characters to take over & to get a really solid way of generating cash for themselves between adventures as well as prime opportunity to steal several space craft. Given you've got an astronaut class from BCK's this is prime opportunity to get the Earthers to Cha'alt or Kanahu.
Megadungeons don't exist in a void & these are often the source of great evil in the universe. Think about it, there is an appendix in ACK's for incorporating supposed heat sinks of evil into campaigns. Power begets power and a place like Cha'alt are going to attract every scum & outlaw from across the planes. Its only going to be a matter of time before Earthers are gonna expand onto some mess like the Black Pyramid.
The question becomes what happens to the rest of humans stuck on Cha'alt. Will the PC's use their growing power for good or evil? There are other options as well a review of the Adventurer, Conqueror, King rpg from Green Skeleton gaming from 2016 gives several opinions on the game in the comments section. These are insightful for the sort of book keeping involved. The wandering gamest blog gives some very disconcerting opinions about the bookkeeping involved in such a campaign also from 2016.
But then gives several house rules & PC class options that are pretty sweet?!
There's also the option of running such a science fantasy campaign with Castles & Crusades which believe it or not is possible.
But as always I'm gonna have to wait to run such a campaign until DM Steve gets back into town. Or is this my only option? Watch this space for updates on this.