GURPS and MA state that the Concentrate penalty within a grapple is -infinity. You're dissatisfied with this, so I think what makes the most sense is to change the penalty to something else. A penalty equal to your opponent's strength is one idea, although as discussed in the other thread, it's usually the same as -infinity unless you have some way to get a bonus big enough to cancel it out, which is why I advocate All-Out Concentrate with a big bonus.
Honestly, I'm not sure why the Magic chapter included grappling in the list of things you make a Will-3 roll to concentrate through... I guess it's for if I'm a mage, an orc comes up and grapples me, and my friend shoots the orc to death before my turn comes up. In that case, technically I was grappled, but I never had to choose a maneuver. Basic Set is very clear:
Quote:
If you have been grappled, you cannot take a Move maneuver unless
you have at least twice your foe’s ST. Aim, Feint, Concentrate, and Wait
maneuvers – and ranged attacks – are completely impossible.
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Finally, if you're considering the "degree of physical involvement," you should probably clarify what you mean by that. It seems to me that spells with a verbal component shouldn't be penalized any more than other Concentrate maneuvers unless the head or neck is grappled, and spells with somatic components should be impossible if the arms are grappled, but possible (but penalized) if the torso is grappled.