What's wrong with a rounded end mortise and a rounded ended tenon that fits into it? Just because we made them square ended in the past, doesn't mean they have to be now. I use a Leigh FMT Pro jig to make rounded end mortises and rounded end tenons, they fit together perfectly, and the joints are as strong or stronger than square cut mortises and tenons because the fit is so much closer and more precise. I've had my FMT for about 10 years now, and would never go back to the old ways of making square ended M&T joints. I've never used a Woodrat, so I can't comment on it, but I see nothing at all wrong with curved end mortises and tenons. In fact, a square ended tenon can be used very successfully in a round ended mortise, if the tenon perfectly fits the flat sides of the mortise and the curved ends of the mortise are left empty. The spaces hold excess glue very well.
Central North Carolina