FWIW, I sent this feature request in December, and received confirmation that something like it was already in their request tracking database:

Save workspace as bookmarks.

I often end up with way too many windows and tabs open, and would
like to save the URLs and start fresh without either making a new
workspace or bookmarking them all individually. The whole workspace
could easily be transformed into a bookmark folder, with one
subfolder per window, for later sorting.

In fact, why not just manage workspaces as bookmark folders in the
first place?