Crits don't work well, from experience, as a pinning tool.
You could try linking the crits to a contested MM, just brainstorming really.
Maybe, the initial attack does a crit, then a contested MM; the crit level could modify the difficulty. Then you need to get a total of 100 percent over multiple rounds to pin your opponent.
eg
Sweep does a C crit
Attacker does a routine MM and gets a result of 50
Defender does a Hard MM and gets a result of 20
Defender is 30% restrained.
Next round, MM's continue.
I think most roleplaying games struggle with grappling and pinning to an extent.