Yes you can. SQ passengers have been doing this and clearing immigrations at T3 to use the Silverkris lounge there even if they are on a SQ regional flight departing from T2. The Silverkris lounge in T2 are the pits. The only problem is that the automated gates may not work with your boarding pass for a flight leaving from another terminal, you may then have to go through a manned immigration counter.

With the MH check in moving to T2 as of today, I would then check in first at T2, wander around the Jewel, before walking or hopping onto a Skytrain to T1 for the OW lounge tour. Sadly the CX lounge in T4 is no longer an option since the transit bus from T4 only stops in T1 and T3.