So the idea is as follows:
Dwarves are grimy little buggers. This is a true fact. Always covered in spatters and smears of various sorts of blood, mud, dust, and if you're unlucky, contact-necrosis FB extract. This is not conductive to Good Times. So here's the idea:
VXXXXXV
VXXXXXV
VXXXXXV
VXXXXXV
VXXXXXV
VXXXXXV
OXXXXXO
Where X is floor (constructed out of a properly valuable metal if you can afford it, smoothed and engraved if not [I recommend leaving a few spaces unfloored if you're particularly artistic and want to construct, say, statues. If you're smoothing and engraving, though, go ahead - it'll only add to the value]), V is a channel, and O is a two-z level deep pit to collect water. This is the actual meeting area, where idle dwarves will congegrate to be cleansed (you could combine it with a dining room - dwarves that go there to eat will be cleansed as well, get happy thoughts from mist, possibly from admiring statues, AND get a happy thought from eating in a high quality dinin' room!). You could, theoretically, make the room as large as you want, but a smaller room may be more work effective.
Now, as for the Z-level above the meeting area:
HXXXH
XXHXX
HXXXH
XXHXX
HXXXH
XXHXX
Where X is a floor (natural) and H is a hatch (all hatches hooked up to the same lever - for extra effect, construct that lever in the meeting area proper, so that even the idler who goes to pull the lever gets a nice, fresh shower). Fill this area with about 2/3 7th levels of water, and pull the lever (time it so that there's a lotta idlers in there to minimize effort). If there's lots of natural rainfall in your embark area, you won't have to fill the pond from above at all (unless you're in one of those odd scorching/high rainfall areas...whassup with those?), but if you channel all the way from the surface, you're gonna want to construct walls around the hole, of couse. (If you have one of those fancy mods with raids of flying creatures, you could construct traps on top of the walls...maybe a three-wall thick layer of protection around the hole, a trap with a wooden cage on top of every wall...and just to be completely safe, you could construct grates over the hole. Metal ones, if possible. Boy, am I paranoid enough yet or what? Uh, where was I...)
If you go for the heavy rainfall option, make sure you pull the lever in time. I can't imagine that being flooded into those pits will make your dwarves very happy at all. Speaking of which, if you're NOT going with the rainfall option, those pits are there to collect the water that gets flooded into the channels (somebody tell me if you're gonna need to construct a pump in front of the hole to maximize collection). Just fill the pond room to the appropiate level, set the ponds above it to inactive, pull the lever...then when your dwarves are clean, activate the water source zone above the pits, set the pons above the pond room to active again, and wait for the next batch of filthy peasants to come along. (Hey, the pond-filling and the possibly required water pumping provides work for the masses - and it'll toughen 'em up. A little. Probably.)
Mister Always Bathhouse Design Enterprises is not responsible for any drowning of small pets and children that may occur during use of the Combination Meeting Area/Sprinkler Bath. Mister Always Bathhouse Design Enterprises does not take responsibility for the odd fancy modded flying invaders that manages to circumvent the thick layer of traps and break through a grate to slaughter your innocent civillians. Neither do we accept blame for the unlikely but possible spreading of syndromes. And if you want to fill it with lava, well, then it was your idea from the start.