Does anyone know if it is possible to create a DNG file with more than four channels that can be read by ACR in Photoshop?

I don't see anything in the DNG specification that precludes this. However, the DNG SDK has kMaxSamplesPerPixel set at 4. After changing this along with the fAnalogBalanceData and fAsShotNeutralData array sizes in the dng_validate example, I can now create multi-channel Tiffs (using libtiff) that dng_validate accepts but Photoshop won't read other than the thumbnail. The "Could not complete your request because the file-format modules cannot parse the file" message is not very helpful.

Any suggestions?

Why are you trying to do this? (If there is a good reason, Adobe might lift the limit in its code.)

Actually, I had assumed that ACR and the DNG_SDK already had fully enabled support for DNG until I tried to use them.

I am working with several six-channel cameras designed for museum imaging (and a seven-channel camera on the way). With the additional imaging bands we can accurately render images across different illuminants, accounting for color inconstancy of the original. It would be ideal to store the images in a publicly documented format that can be opened with standard software for colorimetric rendering.

Many proposed spectral image formats can be implemented within the color processing framework laid out in DNG. Lifting the color-plane limit in photoshop would be very helpful to the color community trying to implement practical spectral based imaging methods.

Also, the DNG ColorMatrix tags along with the OECF and SpectralSensitivity tags in EXIF-2.1 solve many of the goals laid out be the CIE TC 8-07 Committee on Multi-spectral Imaging.

If you're interested, the research group I work in at the Rochester Institute of Technology has a website documenting our work, www.Art-SI.org.