We've talked about doing a 9x12 film holder. Frankly, even if we started the project tomorrow, it would be January before anyone saw a holder. (No, that does not constitute a product announcement! We're still evaluating it.)
The good news for you 9x12 aficionados? You can modify a 4x5 holder in about 15 minutes. Here's a short video: https://youtu.be/-o0Io9dmDgc
You can download the drawing here: 9x12 film holder mod.
Several notes: obviously, once you modify the film holder you can't use it for 4x5 film. Also, we assume you know how to cut and form plastic. If you don't want to, or don't trust yourself with a knife, you can probably find a model railroad guy who could do this in his sleep.
One more disclaimer, since we don't have a 9x12 camera, we haven't actually processed any film with it. We also suspect that there is some variation in the actual size of "9x12". You should measure the film you're planning to use and adjust the modification if needed.
And yes, extra film holders will be available on our website as soon as extra parts arrive from the factory.