No, it isn’t easy to determine the variance explained by MDS (which SSH is) style ordination programs. MDS-style programs don’t use variance in their operation. In some cases, the MDS axes are rotated to ‘simple structure’ which usually means that they are rotated to maximise the variance on the ordination axes (eg VARIMAX rotation is common).
I don’t use Principle Components in PATN (as it has too many traps for the unwary), but you can easily export the SSH vectors from PATN and import them into any standard stats program and determine the relative variances, but the result will not have axes that in any way correspond to the SSH axes. SSH simply aims to get the maximum information in the selected number of axes.
Hope that helps.