I don't think it would fit on an SD card without sticking out (at least on the 900). You might be able to do something via bluetooth (or even flash an IR LED at the camera).

I don't think the Honeywells do SPI (which would make it easy), but do I2C - I have the HMC5843 talking to various Atmel chips (the new attiny45 in tssop might fit in the uSD slot though). It could go via bluetooth as well though it would probably be a bit big.