Every rule has an exception, and this is it (at least on a de facto basis). the departure concourse of T1 and T2 are joined together and you can wander freely between the two. This is because T1 and T2 share a few gates (the ones in the high 30's and 40's). It's a longish walk (and not really signposted) but you can head over to T2 and then back to T1 without any problem.

Unlike the old situation at T3, T1 and T2 gate areas are set up to keep departing passengers and arriving passengers separate. So there is no security issue with letting you in. I'm sure they could try to deny access if they really wanted to though (and in any case could doubtless deny you entry if the lounge is too full). Given the 15 minutes or so of walking time involved, the number of people involved can't be all that great.