I haven't seen the entirety of C++'s feature set added to an HM system, but even OCaml has an OO system that's fairly complete. Many new functional languages are adopting Daan Leijen's extensible records approach which can model OO fairly well. http://research.microsoft.com/pubs/65409/scopedlabels.pdf

All of these are extensions of ML that are shipped into production today, so it looks like more than just a theory at this point.