Anyone can build anything really, so long as it conforms to the bitcoin protocol as implemented in the other clients.

So yes, you could probably write some sort of crypto-system that allowed continuous authority for a signed-address to be given a certain amount every so often, on request.

You wouldn't want to right now though, with the volatility in the value we're seeing right now you'd be a fool to agree to pay the same amount for something next month as you do this month.