Storing a column containing a JSON-object before save

Hi all,
I've got a text column containing an object (or rather; an object structure) that's been serialized to a JSON string. I'm currently just re-serializing every time I change something—not very nice— so I'd like to do something…nicer. I.e. preferably, cayenne would know that something within the structure has changed, and take care of re-serializing for me (or let me know I need to re-serialize before commit).
I could re-serialize everything before every commit, but for performance reasons I'd prefer to perform re-serialization only if the structure has actually changed.

First thought was creating a custom ValueObjectType and just model the type directly, but it seems like a bit of an overkill. Or perhaps it's not?

Grateful for any suggestions.

Cheers,
- hugi