What is a fair metric for assessing the citation impact of journals across disciplines?
There is no such index.

Publication and citation standards vary significantly between disciplines and even sub-disciplines.  Without direct, deep knowledge of the standards in each community, it is simply impossible to compare impact of a journal in field X with the impact of a journal in field Y.  (Eigenfactor's extraordinary claims to the contrary require extraordinary evidence, which they don't provide.)

Moreover, it's not clear why you should even try.  Any judgement about the relative importance of Journal of X versus Journal of Y necessarily requires a prior judgement about the relative importance of field X versus field Y.  HC SVNT DRACONES.