Papa Loko is huge in the assogwe lineages; does he exist in tcha tcha/deka, in makout houses, or even "hotter" secret societies? If so, what role or function does Papa Loko play in these non-asson lineages and houses that do not even carry the asson?
Yes, Loko exists outside of the asson lineage in all other lineages, as far as I know. I do not believe he appears in Vodou-adjacent rites, but I could be wrong about that.
What he does and how is treated in lakou that don't utilize the asson is as varied as those lakou are. Each lakou will have their own particular way of dealing with Loko, if he appears for that lakou (since non-asson lakou often have very individualized communities of spirits). In asson lineages, he is never seen in possession but in lakou otherwise...he may come to eat.
For example, in one of my husband's bitasyon that does not utilize the asson, Loko is known to come down to kill his chicken and then eat it once it's prepared. You will not see that in an asson lakou.
His role is also super varied and will depend on where you go. A lot of folks know him as a expert doktè fey, which reflects his place in asson lineages as a master healer.
So...a lot of it depends on where you are and who you are talking to!