A thousand-and-one things a clever boy can do with an MFS!
Any solution depends, obviously, on what equipment one has access to - and this probably won’t help Joe with his particular arsenal of routers…
But for someone to do this with limited resources… using the three(?) other leg components, or any other spare stock that is also 1.625” thick, there should be enough material upon which to lay a rail/straightedge/guide-of-some-sort (or possibly two, one either side of the dado line); could the dado then be made with a small palm router with a tilting base?
Assuming that any such tilting base would not also be plunging, it would mean beginning with the cutter already inserted into a pre-drilled hole; sounds rather awkward but with some care taken, could that work? My experience with routers is limited, so am curious to know if such a procedure would be advisable, or would it be asking for trouble?