Convolution Reverbs for ADR
 

Been reading up on convolution reverbs like Sony's Acoustic Mirror and it occurred to me that these would be a great tool to use to add location ambience to studio-recorded ADR dialog. Has anyone used them in this way? If you have, how do you go about capturing the "impulse" or reference file while on set?

been doing it for years.
You can record ambiences with any percussive object such as a ruler on a table, a hand-clap, etc, but all the impulses we've recorded for Sony, Cakewalk, and WAVES have been done using a starter pistol.
Put the mic in the position of the audience perspective you'd like to create. turn on Record. Fire the starter pistol from the position of the sound source, such as a stage. Drop the resulting wav into Mirror and it will convolute it for you, removing the audio and leaving you with the ambience.

If one is slating the shots, how about lifting out the sound of the sticks hitting together and using it to set the reverb?

The problem with hand claps, slate claps, pistol shots etc is that they are not
"impulsive" enough (a true impulse is not a physically realizeable thing). In the olden days they used electric sparks to appoximate an impulse. Today it is more likely that a white noise source (impulsive auto correlation function), pseudo random noise source or swept sinewave source will be used for measurement and the room's impulse response derived mathematically from the measurements (of amplitude and phase). Once the impulse response is available sounds can be made to sound as if they were recorded in that room be they speech, noise or anything else. Filtering white noise with the impulse response of the room should give something like the actual ambience. Speech convolved with that impulse response should sound as if it were recorded in that room (provided that the speech going in was recorded in a dead room) etc.

White noise was used for the Sydney Opera House samples for WAVES, but to my knowledge, that's the only place white noise was used.
I'd guess we've recorded perhaps 10,000 or more ambiences for libraries in use/sold by Cakewalk, Sony, WAVES, Apple, and at one time, Qup Arts. All were acquired using a starter pistol, and to date, I've not received one complaint/letter/email from anyone about it.
Would/could white noise be better? Potentially. It's also a lot of hassle to set up. Hassle factor doesn't guarantee a better result.
Sometimes impulses that come from non-percussives work exceptionally well too. For example, creating a convolved verb from a full snip of a song's hook and refeeding it as a convolution sounds awesome. Lotta hassle, but sometimes that pays off too.
It really depends on if you're after making a creative point and final product or if you're intent on being an engineer that plays in the lab and never ventures into the sandbox.
FWIW, I'm a mathematical idiot, but I've created some exceptionally spectacular reverbs in locations ranging from volcanoes to mineshafts, opera houses to bridges, toilet paper tubes to 12" PVC pipes, and other unique locations/devices. We even made some very unique convultions using a Cooper tube being fed pitched-down kick drums.

For small spaces, and conditions where the source is fairly close to the mic, you can use a balloon pop, rather than a starters pistol. One advantage is that nobody calls the cops on people for popping baloons.