Ok, so by "Pilot G2" you do mean the G2 Mini refill, right? I know some Fellhoelter pens use the Mini, but I'm unaware of any that use the full size G2. He also makes ones that use a Parker G2, but that's apparently not what you mean.
Unfortunately, as you've found out, you are
very limited to what you can do.
Most bolt pens can swap pretty easily between the full size Pilot G2 and similar plastic gel pens like the Pentel Energel, Sharpie Gel, Uni Signo, etc, with little or no modification (or spring swapping). One rather crappy option is to use a gel pen of your choice until the ink is low enough that you can trim it to the length of the G2 Mini. I'm partial to the Energel needle points myself.
A better option is this:
Check out Adapter: Pilot G2 Mini To D1 Mini by Tofty2 on Shapeways and discover more 3D printed products in Maker/DIY.
www.shapeways.com
It's the adapter recommended by Tactile Turn for their mini pens.
I haven't used the adapter myself since I haven't purchased a pen that uses the G2 Mini, but I have used a number of D1 refills. The D1 is the standard used in multi-pens, and fortunately there's a pretty wide variety of different refills available. I've tried several, but not nearly all. The Ohto needle points are mediocre, but I do like needle points. They write smoothly and evenly, but slowly. The Schmidt Easy Flow are globby gloopy garbage. The Monteverde Soft aren't as bad as the Schmidt, but they're not great either. The Zebra gels are excellent. The Uni Jetstreams write the best. And finally, the Fisher "Universal" refill is a D1 size when you break off the extension bits. Writes like other Space Pen refills, which could be good or bad or simply adequate-but-universally-useful.
The D1's obvious drawback is the small ink reservoir, but it's not like it's going to be much worse (possibly better) than the G2 Mini's gel. I found that the 0.4 Zebra gel and the 0.5 Uni Jetstream actually last quite a while if all you're using it for is notes or signatures and whatnot. The Jetstream in particular will make your $500 pen write like it's a $500 pen.