This post might be useless but I thought I'd give it a go.
It's mainly meant for *G (or even *S) who managed to get their status perks even if they did not credit on A3.
Let me start with my experience from yesterday, February 8th, 2012.
I'm A3*G and I was flying on SQ406 from SIN to DEL at 02:30AM... This kind of flights are awful and I wanted to be able to use the SilverKris Lounge (*G Lounge) in T2 at SIN to get some drinks and snacks.
I am crediting all the miles on SQ (I'm SQ*S). I show up at the lounge desk, make a super polite greeting and smile and ask politely "excuse me, I'm *G on another carrier (show him my A3*G card) but I'm crediting on SQ (show him my BP with SQ*S marked on it), would it still be possible to use the lounge? It's so late and I just want to get some refreshments".
"Mhhh usually you cannot do this Sir, but as an exception, I will allow you to use the lounge, enjoy!"
So here you go, I got in, said a big thanks and managed to take a little rest before boarding that 2:30AM flight.
I was expecting a refusal, we all know how Singapore staff are very very "by the rules" people. But hey, if you don't try, you won't know
The lounge was pretty empty at that time, which might explain why he did this exception ^
As for boarding the airplane, I always show them my A3*G card to board when they call the *G passengers. Especially on "specific" flights where I know that if you get in late, there is absolutely no chance to get your bag in the overhead storage...