Ian,
Sorry i didn't understand what you meant by Retaining the reduced size, as the reduced size is only decided at the end of the process ?
Can you please explain more, sorry for sounding dumb, i,m trying to get my head around it,
Perhaps the disconnect is that you don't use the Uploader system built into Photo Mechanic?
Basically you select images and bring up the Uploader from the File menu. You choose the type of upload you want to do, things like FTP, PhotoShelter, Zenfolio, etc. There are a bunch of options in the templates, ones that let you scale your image to fit a box, convert to sRGB, set JPEG quality and file size limits. These operations cause your original file to be processed according to your settings and the resulting image is uploaded to your service. There is an option to save the uploaded files to a specific location or to a subfolder that will be created where the original image resides.
So in your case you would edit in Photoshop, using your action. Return to PM, browse the folder where your action saved, select all, set your settings including the option to save the uploaded images and you'd be done.
Outside of what I have suggested above, there is no facility to watch for new images and then perform some sort of save as operation on each of them. We do offer a feature that will watch a folder for new images and Ingest them--Live Ingest, but it does no image processing.
HTH,
-Kirk