Jmol already supports isotopes in MOL, XYZ and (limited) PDB. I just found an example of CML file (from PubChem) where isotopes are specified by simply inserting a string like
isotope="14"
inside the <atom tag, just before the x3= parameter
This should be easy to implement and would enhance isotope support.
[14C] acetate, 3D CML format
Hmmm. sorry, not from PubChem but from a Nature Chemistry article. It is 2D though:
http://www.nature.com/nchem/journal/v1/n1/cml/nchem.100-comp7.cml
The attached file, [14C]acetate, could be used as test system. It is 3D, taken from trunk/Jmol-datafiles/cml and edited by me to insert the isotope field.
version=11.7.32_dev
# new feature: CML "isotope" and CML "partial12" bond order.