Well, if Jason needs a disguise and/or weapons, I'm almost sure that Bruce and/or Damian keeps random bat-gadgets and spare domino masks/outfits/stuff hidden around the manor and if Bruce is the one doing it Jason would probably already know where some of them are. Not that domino masks are actually very good disguises which has always confused me. 

On the note of it causing a LOT of questions for them to suddenly switch I was considering that too... although Bruce's adopted daughter just slid down a railing while calmly rattling off orders and assailant descriptions to the rest of the family and giving Damian instructions not to let anyone die, so I'm thinking Damian running off to a 'panic room', quick-changing and then having Robin show up from outside via secret tunnel or something a few minutes later might still have been less notable. Maybe we should just have them 'courageously' help with the evacuation while slipping off in the confusion then come back after most of the guests are gone?

From Xtreme's stuff so far it seems more like Freeze just happened to choose Wayne Manor as bait because it was a huge publicized thing and Bruce Wayne is Gotham's favorite son so wrecking it all would cause despair/be obvious bait for Batman.