I think what I’m after might not be possible. I like the concept of the sheet document type in DEVONthink, and I’m happy to see there’s a handy “URL” data type for columns already. But I wonder: is there any way to make such a column work more like a web-browser or Markdown hyperlink? To wit, is there any way to have a bit of text displayed in form/table view such that clicking it (or Ctrl/Cmd/something-clicking it) opens the URL? As it is, the URLs are always way too long, and as it seems the sheet doesn’t remember the column widths I assign, it becomes very unwieldy to work with on a regular basis.
It seems like the sheet is an attempt to provide a sort of minimal, flat-file database offering in terms of rows and columns, so maybe I’m asking too much. Or maybe that’s an opportunity to add a hyperlink column type? 
Cmd-clicking on the URL opens it in a new tab.
What about using another column showing additional information? Alternatively you could store links in rich text fields.
The main problem with that is that any URL column immediately eats pretty much the whole screen because most of the URLs I need to store are quite long. It wouldn’t be so bad if the sheet retained the widths I assign to columns, but that doesn’t work.
I didn’t realize there was an RTF option for fields. I missed that. I’ll see what I can do with it. I’m not even sure how to copy/paste a hyperlink properly (or format it properly) in such a field, but perhaps some trial and error will dispel that bit of ignorance.
The easiest way to edit rich text fields is probably by using the form view.
I tried your suggestion, using RTF options for the fields instead. The good news is that I can indeed paste URLs in RTF from my other Markdown documents there. And I find I can Ctrl + Click the link in form view to get the option to open in an external browser (viz., Safari). So thanks for that!
My one remaining problem is that I don’t see any way to access the links in table view, and that’s the view which is honestly most helpful. It lets me see a lot of records at once and pick exactly the URL I need. Is there any way to get the links shown there to work as links and not just link-styled text?
After clicking on the cell this should be possible actually.
Aha! I confess it never occurred to me to try clicking it like that when the control had clearly switched to its “edit mode”. That does work. Thanks. Is there any chance you might get it working without having to (1) click the field to activate, (2) click in blank space or use the cursor keys to deselect all the text, and (3) then Cmd + click the link? A simple Cmd + click (or Ctrl + click or whatever) to the inactive field would be a much more direct and simpler UI process.
Cmd-clicking on links in active rich text cells should actually open the link in a new tab.