It's been a loophole forever. You have to be in the organization but not on the roster. K-Rod was not on the roster that year he made his Angels debut. Technically it requires an injury but even that is a huge loophole [for instance right now Moises Chaice would provide the opening].
In this case, sure, he'd have to be under contract and assigned to a minor league team [or even just Clearwater]. But that's all.