Hi Gippie,
I can't answer the second part of your question (i.e. if they don't have a BM), but I did just do this today, and did insert the soap pellet all the way until he'd swallowed it up. He had a pretty surprised look on his face, let me tell you! But bingo, he started passing gas almost immediately, then straining. My poor little guy.... He was crying and having such a hard time. He was trying so hard that he was making himself pee too.
In the end I combined advice, set him on a towel, took off the diaper and got ready for mess. I held his feet up in the air, and using a cotton bud, I nudged the little pellets out as he tried to push, otherwise they would disappear back in. We got about four inches of poop out that way (as well as the original piece of soap).
I then bf'd him as it was time for his feed. I did the soap thing first in case he needed comforting after. I felt him straining again, and when I checked afterwards there was this entire softer dark mass of poop in there -- the prunes chaser that I sent down two days ago!!
Hopefully we're back on the right track now. But I had to say that the soap trick totally helped us out in a pinch.
I never thought that being a Mom would see me a) doing some of the things I do or b) talking about them in such explicit detail!
Katy