The only thing that would put me off is the weight (as mentioned above). If I were to have a go at something like that (which by my nature, I would), I'd think of putting a suspension mountainbike rack on the back - the kind that attaches to the seatpost. My main concerns would be the material of the tagalong. I'd always go for steel as its fixable (I don;t know, but maybe it is steel anyway), but I imagine with something like that, you'd be looking at straight gauge, heavyweight stuff. If I were to do it, I'd look also at swapping the wheel out for something lighter and better quality, and changing the tyre for a slick.
It's doable, and it would be an interesting project, but I'm not sure the results would compare favourably to a dedicated trailer.