Hi Myles,
Unfortunately Photo Mechanic does not support any API mechanisms. There might be a possible workaround if your scheduling app is able to write the IPTC metadata updates to an XMP file. Photo Mechanic can load this XMP file into the Metadata (IPTC) Template to batch update the selected files. If you need to apply individual updates to the images, then a Code Replacement file could be created and used with the Metadata (IPTC) Template. What are the export capabilities of your scheduling software?
Thanks,
--Bob