You should be able to do this, remember gauges are just DLLs in the same memory space as your DLL and the sim. Look carefully at the macro that help in defining gauges, these macros defined in "gauges.h" should help you understanding how the gauge DLLs are built and how to access their data.
For any gauges that utilize a separate thread to do rendering, you run a severe risk of a memory collision of sorts. Also, only gauges that use an HDC (defined by the IMAGE_CREATE_DIBSECTION flag) actually have an HDC at all.
Thanks gents! I've got my own DLL gauges going, more as an experiment I was looking just to read and copy the pixel data from those HDCs so that may avoid the memory collision but I really appreciate the feedback!