I am not an employer, so I don't know if that would be an effective strategy. But I DO know that the few times I actually were happy to write a cover letter, I

I am not an employer, so I don't know if that would be an effective strategy. But I DO know that the few times I actually were happy to write a cover letter, I was really interested in the company. So how do you filter for that? You cannot make the cover letter required, because as you said yourself, that doesn't signal anything then. Anyone can write a decent cover letter if forced to. So the best way is to make it optional. If you don't get enough people writing cover letters with that strategy, you can still change it and look at applications without cover letter. Or you just use the cover letter as one of many signals in addition to the resume.