Just a minor issue:
Looking into the output of "objective" of an MxFIMLObjective, I find that the covariance matrix is names "covariance" but the means vector only has a generic title like "untitled4391". For consistency, I suggest to name the means vector "means".
compare this output:
> model@objective
MxFIMLObjective 'objective'
@covariance : 'covariance'
@means : 'untitled4391'
@vector : FALSE
@dims : NA
@thresholds : NA
best,
Andreas