Just to make sure that I understood what you want to do here: You are on the editing page of that work and are using the inline search to find its recordings, right? And then you are adding a “recording of” relationship for each matching result and would like to be able to select all matches at once to speed this up?
Have you already tried the above userscript to add recording-work relationships? I think it is far more comfortable to batch-add works to recordings from an artist’s recordings page.
But for relationship types other than “recording of”, I agree that it would be nice to relate the currently edited entity to multiple other entities at once. Coincidentally I have created a bookmarklet to achieve this just two days ago 
Its original purpose was to allow adding multiple entities of the correct type as “part of” the currently edited series, which is a really dull task for these audio drama series with dozens of episodes (my old workflow was to repeatedly open a new relationship dialog and drag&drop the RGs from another browser window).
But I just tried to use my bookmarklet on a work’s edit page, fed it some recording MBIDs and it just worked out of the box. So I have gone ahead and simply renamed it and rephrased its decription 
Since this is still a bit rough (it performs no validation of entity types and always uses the default relationship type) and it is based on some work-in-progress code I have not yet published it in my main GitHub repository. It accepts any text which contains MBIDs as input, so you can feed it with entity URLs or enable the “Copy MBIDs” button and use its results which you can obtain e.g. on a search results page.
Apologies if my answer is too much off-topic, but I thought I should mention my bookmarklet here since it works best in combination with @yyoung’s fantastic userscript and @aerozol basically asked for it 