No, I don't, as I am not very familiar with the algorithms Infer.NET implements. I will have to do some reading.
I am particularly interested in implementing Hamiltonian (or Hybrid) Monte Carlo (HMC) and related algorithms. Unlike other MCMC techniques, which scale proportional to ndimensions**2, these algorithms scale proportional to ndimensions**1.3 or ndimensions**1
which makes them much more feasible for large problems than other MCMC algorithms while remaining very general. HMC is not very complex and I've coded it before, which is why I was interested. I am sad to hear it is not possible to define a new algorithm.
Thanks for the information.