Hi Dave,
No guarantee on this but you could dunk it in water at about 120 degrees F for a couple of seconds, the idea being that the outer plastic will possibly expand more quickly that the innner section threads and so loosen the grip.
You could roll it up and down between the palms of your hands for a minute or two, I have no idea why but this does sometimes work on stuck plastic barrels, it worked on a Parker 17 once and on a couple of others.
A couple of pieces of rubber sheet or cut tube to give you a better grip might help but be careful, I have no idea how strong the plastic is on Hero 100s.
WD40 whilst useful for some things isn't a good thing with most pens, whilst it is mostly a silicone lubricant it does have petroleum distillates in it that are not good for some plastics, just a word of caution not a 'Don't do it at any price' prohibition, after all it is your pen, besides not only will it make the joint slippery it will also make your hands slippery.
Sorry not to have any better ideas, best of luck,
Cheers, John