It used to be you could place slabs on top of light sources, and then due to the transparent nature of the slab block, it would allow all light through creating a hidden light source and a glowing floor without any visible torches or glowstone.
The lighting code's been changed now so that slabs are not transparent from the bottom. So this solution is out.
The only other solution I can think of is to buy a second minecraft account and leave it logged in and in that room at all times.
EDIT: Oooh, another solution is to create a rail system underneath the floor. Embarking rails teleport mobs (through walls) into minecarts. Have the minecarts constantly run through a grinder to destroy the mobs. Mobs will still spawn, but if you set up the embarking rails with mob detectors correctly, you should remove all mobs from the room all the time.
EDIT2: If you own the server, are an OP and can enter creative mode, you can also spawn in a command block
http://www.minecraftwiki.net/wiki/Adventure_Mode_Control_Block
use the command block (or set of command blocks) with a redstone signal and a pressure plate/lever/button to set the game's difficulty to peaceful for a second, then set it back to normal/hard/etc. This will remove all mobs from the server briefly and empty your room before you enter.
thats 1 i might try[DOUBLEPOST=1355367832][/DOUBLEPOST]you can put them on multiple faces btw