There area a number of query parameters already allowed on redirect specials, such as: uselang, useskin, debug, and safemode. These are currently taken in as query parameter and passed on as query parameter. I suppose the downside is that use of them would require the {{fullurl}} magic word rather than the square-bracket syntax. But maybe use in wikitext isn't the context you're after, since if this is about wikitext, one could presumably follow the the destination and add the query parameter in question as one wants.

Perhaps the use case is for edit summaries. In that case, I suppose adding additional slash components might be difficult to scale once a second option comes along. But perhaps that's okay to leave as obstacle for later?

I'm trying to understand the reason why someone would want to force the recipient to have or not have the "diffonly" parameter set as this is generally a user preference. We may want to avoid creating a culture where one is "supposed" to create links in edit summaries with Special:Diff and remembe to pass diffonly. That seems like something asking for a better interface in the first place instead of asking editors to figure that out and do it correctly.