I can't take credit for this idea - it really belongs to the SJG Forums member Landwalker, who asked me about this. I hadn't thought about it, but after he asked, I thought, yeah, here is what might work.
I think it's a neat idea, and I'm curious how it will work out in play. I figure that if I post it here, it might get some trials.
Defensive Grip and Double-Dagger(‡) Notation
As it stands right now, double-dagger notation on (some) Polearm and Two-handed Axe/Mace weapons makes them sub-optimal choices compared to single-dagger notation weapons. A double-dagger makes the weapon unready after you swing unless you've got 1.5x the ST stat of the weapon. A single dagger on the ST stat merely denotes a two-handed weapon, no re-readying needed.
Part of the reason some weapons get the double-dagger is because of how they're used - held near the far end of the handle and getting a full swing behind them, and how they're built heavily unbalanced to maximize their impact.
But would it be fair to let choking up on the handle in a Defensive Grip mitigate this? Basically, if you hold a great axe more like a dueling pollaxe, can you skip the double-dagger and treat it as a single-dagger weapon?
Here are an optional rule, which has not been playtested, just eyeballed.
Defensive Grip and Double-Dagger Notation
If you use a weapon with a double-dagger notation on its ST stat in a Defensive Grip, treat the weapon as if it has a single-dagger notation on its ST stat instead, for all attack modes. All other effects of Defensive Grip are as listed on GURPS Martial Arts p. 109-111. These include, but aren't limited to, the +1 to parry from the front, -2 from the side, the -2 damage or -1 per die of damage for swinging attacks.
Some examples: If you use a greataxe (sw+4 cut, ST12‡) in a Defensive Grip with ST 13 (1 thrust/2-1 swing), you would do 2+1 swing/cut, but wouldn't need to re-ready after an attack. You'd get a +1 to Parry, but still have Parry 0U. If you wanted to use it in a Defensive Grip and make a Defensive Attack to get rid of the U on your parry, but you'd be doing 2-1 cutting, but could parry freely. That's pretty crappy, but you're essentially taking an all-offense weapon, swing-and-re-ready and trying to use it as a light and flexible attack-and-defend weapon.
If you use a full-sized halberd (sw+5 cut/sw+4 imp/thr+3 imp, ST13‡/ST13‡/ST12†) this way with ST 13, you're doing 2+2 cut/2+1 imp/1+3 imp. With a Defensive Attack on top of this, 2 cut/2-1 imp/1+1 imp but the Parry 0U becomes Parry 0. A dueling halberd is a better choice, overall, if you intend to fight this way - it gets 2+1/2/1+1 in the same Defensive Attack, and that thrust gets a +1 to Parry since it lacks the U notation.
There is a little comparing apples-to-oranges in this, since Defensive Grip gives a +1 to Parry from the front, too, but penalties to the side, etc. It's not a cheap-and-easy way to turn a double-dagger into a single-dagger with no other costs and otherwise match a weapon better balanced for what you're trying to do. But it does seem fair, and you do have the option of using your weapon as intended - for a mighty fight-ending attack, or (for polearms) at reach and in ranks, shielded by friends. And it might make some of those ‡ weapons into a real choice even for guys who like things like parries.
If anyone tries this, please let me know how it works. No one in my games uses a double-dagger weapon (except Raggi, who's clocking over STx1.5 for a Dwarven axe so it doesn't matter.)