We don't have that automated at present. You'd need to calculate variance explained using either
path tracing rules or LISREL matrix algebra. For a CFA with simple structure (one loading per indicator), the % of variance explained in each indicator is the sum of that indicator's squared (fully) standardized loadings. To get the percentage of genetic variance accounted for by the factor model that is akin to what you'd get out of an EFA of the genetic correlation matrix, you simply average these proportions across all indicators. When there are cross loadings, it's a bit more complicated (when an indicator has 2 loaindgs; in addition to summing the squared the loadings you also add in 2cor(F1,F2)).
I should note that, although it is conventional in EFA to use % of variance accounted for to decide on the number of factors, in CFA model fit comparisons are often used to decide on the number of factors.