In reply to SouthernSteve:
Sorry for the late reply! Yes, he's pretty organised, keywording into IPTC/XMP metadata, which is fortunately searchable with Mac OS Spotlight. But even Spotlight's Smart Folders aren't entirely adequate for categories and the interface isn't like a good DAM, since it's just Finder.
I thought about Photomechanic Plus (I have been using PM for a very long time - probably since early version 4 - it's frankly the best app for PJs and anybody with a lot of photos to cull and caption quickly), but I feel even with customisation, it might still be a bit overwhelming in its learning curve, especially with the new DAM in PM Plus. Although even basic PM can search the Spotlight database instantly, which is definitely nice.
I will have a look at the others you mentioned, thanks!