Retrieve album art from web and save it embedded in files

First, if you have not used Strawberry Music Player, it's the best file based music player IMO.

It can query these sources and optionally embed the album art in the media file...

...and the source code for how this is done is publicly available here: strawberry/src/covermanager at master Β· strawberrymusicplayer/strawberry Β· GitHub

I would like to see this feature added to Mp3Tag so that bulk operations can be perfomed like adding embedded album art to an entire collection at once!

You have eventually added pictures to your whole collection if you go album by album - this is the function already available in MP3tag.
The way it is implemented at the moment lets you pinpoint exactly that release that you really have in front of you and you do not have to rely on some kind of automatism with its own criteria what would be the best hit.
If you do not really care then 3rd party programs like albumartdownloader can be integrated as a tool.

@ohrenkino I cannot use AlbumArtDownloader on a Mac, it is Windows only.

Is there another way to retrieve album art and to embed it in the files for a Mac MP3Tag user?

Yes, as described: album by album with a suitable web source script.
For the whole collection with just 1 function call - no, not with MP3tag functions.

@ohrenkino could you point me toward documentation on how to use this feature? I don't see it in MP3Tag's Mac settings and I am not sure where to obtain these scripts you speak of?

See here:

Just to point out an alternative:
I always use this tool to quickly look up cover art online as it yields more and better results than AlbumArtDownloader (for me at least).
As it only uses a website it should work on mac, but I have not tested that it does. Good luck!