Yes, yes I know, the earlier 707s did not have the APU. However in several publications including the original Boeing manual for the 707-300 (which is 600+ pages and very nice indeed) the APU is present.

The APU engine is located behind the main wheel well and the APU control panel is located at the FE station, lower AUX panel. Maybe we can get this gadget installed on some stage, say SP1.2?...