it was really easy to open up the Leopard Imaging LI-IR650 filter switcher with a tiny philips screwdriver. Inside was a glass IR-CUT filter and a clear piece of glass. I broke out the clear glass and cut a piece of developed film to fit in its place. The developed film stays in without any glue.
This filter switcher is designed to switch at 12v, and works applying +12v or -12v briefly. It switches so fast that I wasn't able to catch it mid-switch at 30fps.
The filters aren't matched in density, so the camera has to spend some time re-adjusting its auto settings, but the filter switcher is more successful than I imagined.
Questions and next steps
Researching Filter Glass
I'd like to spec matched filters with equal density so that the camera can switch faster. Alternatively, getting full control of the camera in software so that the settings can be switched with the filters, but I think that's less elegant and likely to be camera-specific.
Connecting to a microcontroller
I have to look at the simplest circuit to allow the filter to be controlled with a microcontroller. I'll probably start with four diodes, but there is likely an IC that will do it cheaper.