Not really a fan of any saline laxative to force a poo while in diapers. In my experience, any saline, like Milk of Magnesia, Citrate of Magnesia, Epson salt, Phospho Soda, or others all tend to produce very sloppy poo if taken in a sufficient dose to cause the great urgency desired by those into such play.
IMO, there are two good options, depending on the time available go the play session:
If you have plenty of time, at least 12 hours, my favorite is a powerful dose of Senna (or Cascara), for example two Maximum Strength Ex-lax pills. This can be combined with a full dose or two of Miralax if you want softer poo.
If you are short of time, say an hour or two, an ounce or two of liquid glycerin, given up high with a 60ml piston syringe and Fr 24 colon tube or catheter inserted gently for about 8 to 10 inches and retained as long as possible… until the laxative makes you completely helpless to resist and poo in you diaper.
Either of these games are best done when you've not pood in a couple of days so there is lots of poo in you colon for a mighty result in you diaper.