So...
I got exasperated with SRing a shiny Fennekin for my Y BQ, so I decided to pick Froakie, my usual starter, and run around on Route 2 until a shiny popped out. After a couple of days, I got bored, so I decided to try and MM a shiny Roselia on Alpha Sapphire.
Not only did I succeed, I got one in 41 eggs. Ghost is Timid; has perfect IVs in Attack, Sp. Def., and Speed; and despite the fact that I was breeding a Natural Cure Roselia, he has Poison Point.
But it gets better.
After hatching Ghost, I went back to Y to see if my shiny luck would carry over between games. Within the first half hour, I captured my first BQ shiny – Robyn the Fletchling. She's Relaxed, unfortunately, and I would've preferred Scatterbug, but you know what they say: don't look a gift robin in the beak.
Update: So, while I was hatching leftover Roselia eggs...another shiny hatched. Lilac (female this time) has perfect IVs in Attack, Defense, Sp. Atk. and Sp. Def., so I'm probably going to be using her on my team rather than Ghost.
Anyway, I'm now MMing Vulpix (on AS again).