If your bitcoin software doesn't have this feature, you can copy-paste the private key of your address to http://brainwallet.org/#sign and use it to sign the message.
Honestly I appreciate your idea of implementing a sign-in method like this.
But I never use brainwallet.org before, is it 100% safe to enter my private key into this website?
I wouldn't say it is 100% safe, but it is an open source software that runs on the client side (javascript).
If your bitcoin address stores significant sums of money, you better download its source and use them locally (instead of using remote web site).
But probably most bitcoin clients allows signing messages, so brain wallet is the last resort.