I think all editors can read in the results of an external program, which might be a better solution. For example, the par text reformatter. Vim doesn't know a thing about it, but it will feed selected text to it, and replace it with results.

So for ascii diagrams, you might have an external program that generates, at least, various kinds of boxes and connectors, then you could edit the labels from the editor. Bonus if the external ascii diagram generator would take labels associated with entities.

Maybe that's an additional feature for Markdeep, or maybe it's a specific itch for someone else to scratch.