Setting the XSI_UNSUPPORTED_METASL environment variable should enable the MSL parser, so MetaSL shaders will show up in the Preset Manager. I believe you have to re-register mentalray.dll too.
So, in a Softimage command prompt:
set XSI_UNSUPPORTED_METASL=1 cmdreg mentalray.dll xsi
I haven’t tested this myself, so no promises 😉
fail. don’t try this at home kids.