Hi Tancredi,
currently pure Python code cannot be injected in the RDF event loop. You can export the data you need out of RDF (after applying filters, selections or other transormations) with AsNumpy and then operate on numpy arrays, or indeed you can use C++ functions.
If fast-jet functions can be called inside of numba-compiled Python functions, then you might be able to use ROOT.Numba.Declare to generate a function that can be passed to RDF, see ROOT: tutorials/pyroot/pyroot004_NumbaDeclare.py File Reference (you can use ROOT.Numba.Declare with arrays too, RVecs are converted to numpy arrays).
Cheers,
Enrico