Hey mate, I live just down the road and go in fairly regularly so hopefully can help with a few answers.
On day of the gig it'll be absolutely rammed. I know Kastro has said it isn't ticketed in the day so fine to just rock up.
I can't remember if there has been a queue in the past - I've never queued on before on Maiden London gig days, but on then again I've been in fairly early. I think possibly there has been, but there's always been a decent churn of people so I don't think it's been too long to wait to get in.
HOWEVER, could be different this time. The 02 is a bit further away (not miles, but couple of tube stops) so people often come, have one then shoot off, whereas the Olympic Stadium is literally a 10 minute walk so I can see people sticking around longer, particularly as the other pubs in Stratford aren't great (I do recommend the Escape which is also a really good pub - not a rock pub as such but def alternative. I'd imagine Roof East will be open too which is fun, especially if the weather is good). The other thing is there'll be potentially 5 times more people than at an 02 show. All that means, I can see a world where there's a long queue to get in that doesn't seem to move. It'd be worth getting down fairly early.
That said, I really recommend you do if you can. They did it up a couple of years ago and done a cracking job, managed to update but give it a rock pub feel, genuinely one of my favourite pubs in London. I always go up on Maiden London days and it's a great atmosphere, there'll be Maiden fans literally from all over the world, and everyone's dead friendly. I've ended up meeting folks from Colombia, Brazil, US, Japan as well as all over the UK.
After the gig there is a ticketed show downstairs in the venue bit (Gypsy's Kiss, with ex-Maiden members from the early days). Unfortunately that's sold out, I didn't manage to get a ticket for that myself. I imagine the the main bar upstairs will be open as normal though. It's got a late licence so stays open 'til about 3 on a Saturday so plenty of time for a few afterwards.
Hope that helps, if anyone has any other questions I'll try and help. I'm sure I'll be up between then and now so if anyone has anything they specifically need to know I could always ask.
If you do make it down on 28th make sure to say hi, I'll be the long haired bloke in Maiden shirt!