I did an experiment the other day. I think it turned out better than expected. I widened the grip-section on my TWSBI Eco and it's great. I was actually fixing another pen that I bought used and broken. The clip was broken in such a way that the cap had a hole in it, meaning the pen dried out in a few hours. It was easily fixed with some Sugru, but Sugru can't be saved once the packet is opened so I decided to try and fix another pen that had been bugging me.
I didn't really think to take pictures before I started but the Eco is a bit of a modern classic. It's well known and well regarded but has a really narrow grip. It was one of my first fountain pens and it's been continually inked with a boring, standard, quick-drying black since I got it. Mine has a rather dry extra fine nib that works well for my tiny writing. But I never really liked the grip, in addition to being narrow the plastic was slippery.
So I put some Sugru around the section and smoothed it out. At first I got a bit too much, I hit the inside of the cap when closing but after taking some off and smoothing it again the cap fits without any issues. The only other thing I was careful of was to not get any in front of the three little bumps, as I didn't want to interfere with the sealing in the cap. I also initially thought that I would have to smooth it out with some sandpaper like I have with some other Sugru projects, but I managed to get it fairly smooth anyways.
I think it took about a third of the Sugru packet and about 24 hours to dry.