So I am all about authenticity... as a result, I always prefer a mushy poo, it's just more humiliating and babyish. Plus it takes a lot less prep. One solution I've found is C02 supposotories, which tend to draw the least amount of liquid into the bowels, but as they create urgency by causing a sensation of fullness, have much the same effacacy as any other suppository. Otherwise, I'm inclinded to agree that really the only laxative that would in any way firm things up would be bulk fiber.
On the other hand, going back to my desire for authenticity, I am a fan of Castor Oil... a fan in that I don't like it. It takes a lot longer than Phospho Soda to work as far as oral laxatives and once it happens it's a fairly miserable, cramppy, oily affair... but that's what real children were given as a common purgative back in the day.
Finally, I will admit I am also a fan of enemas and suppositories, however enemas can be hard to orchestrate into a diaper... the liquid content is so much. Unless you want a truly leaky baby or do a mega-diaper with like double Abenas or Dry 24/7s and a stuffer and try to limit the water quanity. I am not a fan of Fleet enemas, for some reason they leave me feeling bloated and dirty, except the Fleet "naturals" product that came out. When I do use suppositories, I find the solid glycerin is not quite effective enough, I tend to stick with Bisacodyl, which is both what they put in Dulcolax and is the main ingredient in my favorite enema of all, Enemeez Plus. Look it up if you aren't familiar, there is no more urgent feeling than an Enemeeze and within 15 minutes, you'll either be messing or crying if you are still trying to hold it.
Huggies!