Another thing you can try is to make only the front 2" or so a dovetail joint and the rest (22") a regular dado. More or less you use a router to cut the dado just into the dovetail joint (say 22.5") and then cut the dovetail pin in the front section. Then you only cut the tail on the front 2" (or so) of the shelf (or whatever it is). Only you will know that it's not a full dovetail and the dado will slide in quite easily. It's still a strong joint and you get the look you want as well. I believe this is an acceptable way of doing this type of joint as I have read about it in multiple books (if you're going for accuracy or something).
Good luck and do quite a few practice joints. One thing to watch for (that bit me the last time I did this) was to make sure as you cut the tail that you make sure the bit is level on both sides. I did it on a
3" wide piece and my Rouseau router plate is slightly crowned and I ended up with slightly different depths on each side of the tail (or is it pin, I can never remember). Anyway, if I had it to do all over again I'd use my Leigh D24 to cut the tail so it stays parallel all the way across on both sides. My problem in not recognizing it with the scrap test pieces is that they were wider than my final pieces and though they slid in well (so I had the width right) they weren't flush at the front so I wasn't able to pick up on the slight parallel problem with the "shoulders" of the tail (is that the right way to describe it?) until I actually glued in the final piece. It's only about a 16th off on each piece but damn that bugs me. No one else will probably know but I always will ;)
Mike