[Lio]

Does it toggle or is only when the fn key is held?

If it's only whilst the key is held it effectively means you'd have to use your right hand to press the escape key ...which doesn't sound ideal to me.

[eridius]

Esc is always available while you're typing (and usually available when you're not). Holding Fn is for getting at the F-keys, not for getting at Esc (though it can be used for getting at Esc too if you're in a context that normally hides that key).