What I ended up deciding on is a mixture of all three! The base is Mutants & Mazes, the LL-MF integration described in an appendix of Mutant Future. I've come up with a very simple custom character generation system, which I'll post here in sections over the coming days.
The first topic is race. The Carcosa book describes the 13 different races of Men who inhabit the planet, but doesn't mention how a PC's race should be determined, or any differences between the races apart from skin colour. So I thought a random table would be in order, including some little mechanical differences for each of the races.
|1-2||Black||+1 bonus to saving throws versus poison or disease|
|3-4||Blue||+1 bonus to unarmed attacks|
|7-8||Brown||+1 bonus to saving throws versus magic|
|9-10||Dolm||+1 bonus to melee attacks|
|13-14||Jale||+1 bonus to learning rituals|
|19-20||Red||+1 bonus to missile attacks|
|25-26||Yellow||+1 bonus to saving throws versus fear or psionics|
|27-28||Roll again||Gain a mutation*. Additional rolls of 27-28 stack.|
|29-30||Roll again||Gain a psionic power. Additional rolls of 29-30 stack.|
* For racially determined mutations I'll probably use the table in the appendix of Carcosa. Obviously, as I'm planning on using Mutant Future, the beneficial mutations therein will also come into play.
ps. For players in my Labyrinth Lord campaign: don't worry, your PCs are not about to be mysteriously warped to Carcosa! Haha. I'll probably run it as a one shot / occasional sessions, just for fun.