I dont think theres a bantam breed that lays dark, dark eggs. Bantam eggs tend to be lighter(even in dark egg breeds) then full sized ones of the same breed.
There are Bantam Welsummers and Barnevelders plus their are developing Bantam Marans. BUT they don't lay the dark eggs of their LF counterparts so a Bantam Olive Egger could be possible but would need a darker egg ancestor first.
Could you use a standard dark egger over a banty amerucana, and select the small birds, and breed them down in size? It is just a thought.