Yes, in principle all the subspace projections should be applied to the forward model.
In Brainstorm, only linear operations can be reported in this way, therefore I'm not sure about tSSS (if the correction is not the same a different time points, there is no way to handle this correctly with this Projector structure).
In practice, this is not done also because the SSS implementation in Elekta Maxfilter is not documented in the output files...
@John_Mosher @Sylvain: Is there something we should add here?