[danjcla]

The more I look into it the less it seems like templates / the GUI (sans writing or finding some custom module) in general could do what I'm interested in doing.

That is support a fast workflow where I occasionally download a bunch of .mobi files via a web browser directly to my kindle, putting them in / creating the right categorization folder at that time, and then run some program that abuses the Author metadata to allow it to be used for categorization based on the folder containing the file.

It looks like the GUI workflow would be more like this:
1. Download .mobi files to temporary directory structure on machine running Calibre
2. Open Calibre
3-4. Import files into Calibre in batches from each temporary directory
3-4. For each batch, add tag equivalent to temporary folder name
5. Do "send to device" (I keep looking for a sync all button...)
6. Remove files, but not directories, from local temporary directory structure

Or alternatively, maybe download to kindle, then copy over to local using the GUI, then copy back (filtered through plugboard and with template that will put files in same folder hierarchy).

The two GUI options just seem... inefficient and annoying.

Also I have a laptop, work desktop, home desktop, etc., not sure how I'd keep Calibre in sync across all those machines.

But maybe this isn't the right forum for this question? If so please tell me and I'll re-ask elsewhere.