[exim] wishlist: custom hints DB use

Musing on rate-limiting my miscreants, I wonder if it would
be possible to use a hints DB. Possibly a variant of the
${lookup syntax would be appropriate, combined with ${extract,
to pull fields from a returned record. I'm not sure how
best to express the write operations.
The goal is to support storage of optional info, with
customisable keys and data, and ease of ageing/deletion.

Cheers,
     Jeremy