Can I gain access to the Thieves Guild treasury?

Solution 1:

Nope. Your share of 'The Treasury' will show up in the Tribute Chest, but there's no extra loot hiding in the Vault now that you're guildmaster, even if you noclip in there.

Solution 2:

The room should remain locked until you have completed "Under New Management". You know you'll have totally completed the Thieves Guild questline if you have the "Guildmaster's Armor". This also involves completing the 20+ or so radiant quests from Delvin and Vex in "Restoring the Thieves Guild to it's Former Glory."

Once you do get access to this room, however, it is just as empty as Mercer left it. Which makes me wonder where he stashed it...