What complementary observation campaigns have looked at Kepler's field of view?
The Kepler Field had been covered in the near IR by the 2MASS catalog since that covered the entire northern sky.  There was also a Smithsonian Institute program called the Kepler Input Catalog program that observed the Kepler field with the 48-inch telescope at the Whipple Observatory on Mount Hopkins, Arizona in the SDSS g, r, i, and z bands plus a custom filter centered on the Mg b lines.  Also many individual objects studied by Kepler were observed in follow up studies.  So, yes this is a well studied region of the sky now.  Now, however, the Pan-Starrs survey has imaged most of the whole sky with several optical filters. 

After the reaction wheel failure on Kepler, it was repurpose as the K2 project.  In this project, it observes a different part of the ecliptic plane every 3 months.  Catalogs of stars and galaxies in each of these locations are created ahead of time with the EPIC survey.  In addition, there is a SN/AGN search ongoing with K2 that includes simultaneous groundbased observations using various wide field imaging telescopes around the world such as the SkyMapper in Australia.  A workshop on this will be held in February.