I've got a mortising attachment for my drill press, but I think I have used it once. It has bushings in it that make it attach to different drill presses. In my opinion there are only two ways to go...with a benchtop mortising unit, saving you time, aggravation and a lot of set-up with a drill press mortising machine
( I hate changing set-ups like when I think of the shopsmith, great as it might be for some, and considering what you and Reg mention I am leaning toward a single usage tool) or...
Use a drill bit and hand chisel to chop your mortises. I know this has no merit these days, but for me I would rather use a drill press and chisel then any other method. Slow but enjoyable for me.
(Thanks Travis for the sharing of this option, I feel I would not be troubled with this for a non production job. )
In all honesty, I think you will find a ton of drill press mortising attachments out there that just plain never get used. There is a big reason for that. You might want to be smarter than me and look at some alternatives before spending money on something you won't use much.
( Thanks, Shaz )