Thanks. SR555 looks like it would work for this, and they explain how to use it separately from a lock-in amplifier, which is nice. Also very cool that they show block diagrams in their manuals.
Although, given the price, I'm pretty tempted to lay out a board in Rogers RO4350B with an LTC6268-10 (and maybe a post buffer amp), power it from 9V batteries and quiet regulators, and put it in a metal box. Doesn't need to be pretty.
Come to think of it, I should probably run an impedance sweep on the AC millivoltmeter input before trying to hook a preamp up to it. Haven't had the occasion to do that to test equipment before, haha.