Hi Ben,
I'm very curious to see how the MAN methods works for traces in monazite samples. I also wonder if you could just utilize some of your REE phosphates as blank standards to test the trace element accuracy? Of course if you have some chem age standard monazites that would also work, though as you pointed out above, the chem age is somewhat less directly connected to composition, since the chem age depends on the ratios of the three elements (U, Th, Pb), as opposed to the individual concentrations. I've seen monazite compositions that vary a fair amount, but still produce roughly the same chem age.
Again Mike Jercinovic and Julien Allaz are the experts on these questions.
john