No reason you couldn't put multiple targets in the same room:
Wight (for testing undead without worrying about slashing/bludgeoning)
Iron Golem (any 100% fort construct works, but dogs are a special case that are damaged by negative energy, so maybe not dogs)
Halfling (for non-favored enemy testing, and because everyone loves torturing halflings)
Ogre (for testing favored enemies, without troll regeneration messing up your numbers or worrying about DR)
This would let you test a wide array of dps options. Wrack construct and/or smiters? Check. Power Attack vs Precision comparisons for no-fort and 100% fort mobs? No problem. Compare favored enemy dps vs non-favored enemy? Go nuts.
When you enter the instance, you'd get the standard four difficulty options. This determines the exact hit points for all four targets, not the CR, and there would be no scaling:
Casual: 1k hit points
Normal: 10k hit points
Hard: 100k hit points
Elite: [Dr Evil]One MILLION hit points[/Dr Evil]
A million is enough HP to test a full raid group of endgame alts, yes?
EDIT: You know how Shadow Crypt has different colored rooms thanks to lighting effects? Have this instance color-coded similarly, matching the compendium colors. (Casual = ?, Normal = green, Hard = yellow, Elite = red) so that videos would have self-evident difficulty info, as opposed to having to bring up the XP report to show the difficulty. I guess blue for casual?