I have one of those pricey slender sterling H. pens, jtroops, and I've had nothing but trouble with it. After about two uses, it wouldn't retract any longer, and I gave up on it and returned it to Hermes - the replacement didn't work any better.
I'd take mistikat's sage advice and look for a less expensive pen that will fit and will actually work. (I found one at Sam Flax Art Supply. It came with a small agenda, which I had to buy, but I think the total cost was about $10, and I now have a pen that actually works, fits in the loops and won't damage the agenda or anything else in my bag, including the bag!)