From Address on Custom Email derived from an Email Field

We're currently able to specify the To Email as being derived from a user (email field).

It would be great if the same logic could apply with the From Email.

An example:

Knack is used as a database for capturing Defects. You want then the User who created this defect as the sender of the automated email, so replies all go to that person.

I'd really like to be able to control the From field too. Set it to logged-in user would be awesome... 

I agree with Adrian; limiting the "from" address to being known at build-time is extremely limiting, when the email going out is likely to be replied-to (which I think is in most cases), but the person to whom email replies should be send is dependent on what is happening at the time.