Thanks for your answer theymos. I'll bite the bullet and go with a db.

Thanks. I was wondering more about storing this state against the wallet data structures. For example the blockchain has no notion of named accounts and address groupings, but the wallet certainly does.

For example to add a flag or comment to indicate that a transaction has been seen and/or processed.

I know it's possible to move addresses between accounts, which would achieve this at the address level

However, I also want to mark individual transactions that involve the same address/set of addresses without resorting to another state store/ database etc