***When I say, "I've tried it with Doomsday," I meant that I tried to dash over grounded opponents with Doomsday, and it doesn't work. This is Shazam specific.
The crossover seems to work on every character, regardless of size.
I honestly think this has something to do with Shazam's dash animation, but I'm not sure. Shazam's standard grab 50/50 still applies once he crosses over, as well.
***UPDATES***
This isn't possible after every combo -- only after ones that end with torpedo. In fact, it works after just landing a torpedo.
You can crossover the grounded body within 2-3 seconds of hitting with the torpedo. You don't have to sweep in order to crossover.
If done in the corner, you can take one step and be in the corner. If you do nothing, you are put out of the corner. This will probably catch a lot of people blocking the wrong way.