Exactly the same way as SRing for stationary legendaries works. Since all Pokemon in the Friend Safari are at level 30 you can plan ahead and have Pokemon in your party for checking the speed and HP stats in the battle. When I was trying for a Jolly Metang in a Quick Ball I would enter the safari, save, use my capture o-power, run around and hope for a Metang to pop up, if I encountered something else I would run away and keep trying for Metang. When one appeared I would have my Jolly Synchronise Gardevoir in the lead which was also trained to have exactly 1 point less in speed than what a Jolly Metang with a 31 speed IV would have at level 30 (which is 48. My Gard was level 42 with 8 speed EVs which gave 94 speed, I gave it a Macho Brace which cuts it to 47). I would use Thunder Wave and if I went first then I would run from the battle. If I went second then I knew the Metang had perfect speed, I would then use Healing Wish to KO my Gardevoir and bring in my HP checker. I can't remember what I used, I think it might have been a Starly that I bred to get Final Gambit early, but it would have been trained to have one less HP than a Metang with a 31 HP IV. If I killed it then it didn't have perfect HP and I would start running around again. Once I found one that passed the speed+HP check then I would start throwing balls. If the stats after the battle were rubbish then I would soft-reset so my Gardevoir and Starly (or whatever it was) were alive again and my o-power was back to full and start all over again. It was a pretty fun experience and I'm definitely gonna do it again. I'm currently planning to do Golurk since it's the only other genderless / mono-male Pokemon that's only in the Friend Safari and can't be DexNav'd. I might also do Ditto there since you only really need perfect HP for a battle-ready one which is easier to test in the safari rather than DexNav and hope you get lucky...