Some countries have issued circulars regarding such operations. Just for an example, I have seen Indian Aviation Authority has issued one circular that authorizes operators to single engine taxi operations. I want to know whether FAA has issued such circular or not! If yes, which circular is that? Are there any other countries that have issued such circulars?
I further want to know whether an operator can perform single engine taxi in and out operation without the approval or authorizations from the aviation authority.
It really does not matter unless you are issued a ops spec from an authority that governs the issue. How will they find out? The largest limiting factor is engine operating procedure (most have a warm up time before pushing up to T/O thrust). And as mentioned above MFGR limitations, also might be sort of embarrassing if you have to make some tight turns that the aircraft is not capable of on one engine. As long as you know your airport (not authority) it is a great idea!