You get your horde to put something trivial on a neutral AH like the one in Booty Bay and have your alliance 2nd account/friend buy it (with money you have presumably given them). You will lose a small percentage to AH fee’s ofc.
The alternative is to find someone on the opposing faction who is looking to do the same and just to a straight trade. i.e. your high level alliance main giving money to a low level alt of a High level Horde, and them him doing the world of warcraft gold . Again you do sort of need to trust the person, or make the transfers in small amounts, like 50g a go or something.
Buy Guild tabards if revered with your guild. The epic Tabard costs 300gold I believe and you can mail it cross faction and then vendor it for 62g50s I believe. Huge loss ~80%.