If we're talking houserules, I would say the GM might assign a certain percentage of grapple needed to have a 'good hold' on the target first. You might say 50% or 75%. You could accommodate degrees of success to by imposing a bonus or penalty for however much the hold exceeds or falls short of that target number. So if the target number was 50% and you had a 70% hold, you would get a +20 to the maneuver. (Remember that a 100% grapple is completely immobilized, so I don't think you'd need 100%). Then as Spectre 771 says, a check to throw the target; you could do an opposed check too.
One of the things I've felt Rolemaster lacks is some rules for some of the basic combat maneuvers that players are naturally going to try in combat. DnD for example has rules for trip and 'Bull Rush', the latter being an attempt to bowl over an opponent or push her over a ledge. It would be good to have some basic rules for these simple actions in the core rules.