Dirr lists chastetree lilac as a species related to V. negrundo, which he lists as "Zone 6, often listed as 5 but shakily so, does well in southern states." He goes on to say that V. agnus-castus "...is not as hardy as" V. negrundo. Sounds iffy! Maybe in a protected microclimate.
hortster

Awesome Blossom. Thanks much for the taking the time to find/post that info hortster!
CMK

The nursery that I used to work at in this area sold them as "semi-hardy" like butterfly bush or crapemyrtle. Over a decade ago when we had some -10 degree winters they did freeze back. Still think you could plant it in the right microclimate. Can get tree-ish if the winters are kind.
hortster