Just a PS has the X key to flag photos to be deleted, it would be helpful if PM would have a similar feature. Perhaps it does and I don't know about it.
I usually go through a quick round of tagging and then select all the tagged images to delete. It's technically two keys (command-+) but it's still pretty quick. If you already use tags for something else, you could do the same thing with a designated star rating or color class.
Is that what you're looking for, or am I misunderstanding your question?
In Lightroom, the X flag is persistent and means rejected, even if not deleted immediately. I often mark images as rejected, but do not get round until deleted as a larger batch later.
By contrast, flagging in PM does not indicate that anything in particular is intended, just that the image is flagged. This may be positive or negative, and may not be immediately obvious if you flag images for some other reason a letter.
For this reason, I would a support a flagging-as-rejected feature, in addition to multi-purpose flagging.
The workflow in Photo Mechanic is designed to be flexible. I use tags. Other people use color classes or star ratings. If you use color classes, you can label the color classes in preferences to make it clear how you're using them. By default in Photo Mechanic 6, the 8th color is already labeled "trash."
How do i assign the 8th color with a hot key?
Preferences>Accessibility under "Single Key Shortcuts."