I don't think there is a problem using the neutral AH to transport money from one account to another as long as you own them both. One of the purposes of the neutral AH is to sell stuff across factions and that's what you'd be doing. I can't see there is anything "illegal" going on if you own both accounts.

If don't have to spend your gold in a hurry I would try and avoid the neutral auction house taking 15% of your gold you could check both faction AHs for, say, Large Brilliant Shards or Black Lotus or something like that. Buy them on one side and sell them on the other side. You could even make a little profit if you are lucky. The risky part is getting them through the neutral AH since you want to put them up there at a low price, like 1 copper, and buy them at the low price (leaving the neutral AH with 15% of 1 copper). Of course this is more time consuming than accepting a 15% "loss".