theclubsq, it's not impossible as an SL to get a birthday upgrade as my post towards the top of the page is proof. However, I think it depends on where you're flying from and the upgrade will be given only on the date of your birthday.

My itinerary was PER-HKG-SIN with departure on my birthday, however the sector to SIN was on the day after. Was upgraded from Y to J for PER-HKG, but remained in Y (as expected) for HKG-SIN. I may be wrong, but I can't imagine too many elite MPC members being based in Perth which is probably why I was flagged for the upgrade.

Best thing though is to not expect it, then be pleasantly surprised if it does happen.