I'm soft resetting for it on Colosseum. However, I've soft reset for Latios the same way you are doing. What I did was that I just kept flying to Mauville over and over again until Latios was on an adjacent route. Once it was in one of the four routes, I just went to a different one and hoped for it to turn up in the same spot. It helps to already have Latios in your dex, so that you can locate it without actually having it.