I'm working on adding write support. The structure of the encrypted data in 1Password is kind of inconsistent, so it's not as simple as just updating fields in the JSON documents, but I don't think it should be that hard to make it writable.

I've been working on a pure-JS browser extension for Chrome/Firefox which supports two-way sync with existing 1Password vaults via Dropbox that has the ability to add and edit items: http://robertknight.github.io/passcards/

There is no support for folders or tags at the moment in the browser extension, although there is a command-line tool which does have additional editing functionality and works on all platforms that Node JS does.