Well, my plans for Harry are a tad up in the air at the moment. I had planned for him to recover his memories a few months ago, but unfortunately had to take a leave of absence from the game and it became unfeasible to continue with this plot (as was Harry to know that Remus and Sirius were in the Manor he wouldn't have been hiding from them for the last couple of months). I do want him to regain his memories sometime though, as - while the amnesia thing's been fun - I'd quite like him to interact as Harry with the other HP characters. Adam's already offered to return Harry's memories, so he just really has to ask for them back (which he hasn't yet as he's erroneously formed the impression that he was involved in the criminal underworld before his accident).

With regards to the BBQ, I think that I'll have Harry show up and the other HP characters spot him if their players want them to. If you'd prefer to have Harry bump into Sirius sometime later in the game this is fine; but if Harry gets his memories back and realises that Sirius is in the Manor then he'll definitely want to see him (though he could conceivably have his memories returned and not realise that any HP character other than Luna was staying at the Manor).