I had the same problem, and had Astarion stabbing that chest as if it was a mimic to trigger Kagha's quest. While the result was similar, I think that you miss on some of the flavour:
- It's a bit unrealistic that the hunky angry librarion won't hear you destroying the chest.
- I liked the touch of an obsidian stone drenched in blood, to signify Kagha's blood pact with the Shadow Druids. When you destroy the chest, the stone is lost.