I use Stoner thermoset with epoxy and I'm not super happy with it. It's spendy, I have to have it shipped to me, and my silicone mold plugs still wear out pretty quickly.
I don't cast for pens but for other purposes I have long used DROP OUT mould release from Midway Arms. It is a gunsmithing product and works very well. Similar products can be purchased from Brownell's gunsmithing supplies.