Would you offer SRS to a hippocampal metastasis in a patient with ES-SCLC treated with prior WBRT?

I agree with Dr. @Dr. First Last. The brain metastasis is a greater risk to the patient than SRS would be. Whether the prior WBRT used hippocampal sparing or not would not change my recommendation.

I am not well-versed in neurocognitive anatomy. But it is feasible to injure (or remove) 1 hippocampus ...

Yes.

Consider adding memantine to reduce cognitive decline.