I imagine it wouldn't be too difficult. If the ratio was kept fairly high (say 10% of the Ordo as Frumentarii), it wouldn't be exceedingly difficult to distribute the workload. It would be quite easy to gather information on the more open individuals, and the more "closed" ones could eventually be worked out through careful deduction and reasoning. Naturally this is all hypothetical, but I'm just saying it wouldn't be mind-blowingly difficult to do.