Hi Ian,
The statepoint files that contain tally results have been relatively stable for some time now and there are no immediate plans for changes in the coming months. I can't guarantee that there won't be any changes down the line, but generally we try to ensure that changes are "additive" if possible, which is easy with HDF5. Our file formats are also versioned, so an external application like MeshTal Viewer could check the version of the file format to ensure that it is a supported version.
I should also mention that we do have a
GUI application for geometry and tally visualization that handles mesh tallies as well as other types of tallies. The way that tallies work in OpenMC is slightly different than other codes -- a user can "compose" together multiple filters (including a mesh filter) in an arbitrary order, and the resulting tally bins correspond to the Cartesian product of all the filter bins. This does make it a little bit complicated for visualization purposes; you could take a look at our plotter application to see how it's handled there. Anyway, happy to discuss further if you would find it helpful.
Best regards,
Paul