I bought a CX-100 (lever fill) from a flea market recently and after removing the renew-point nib, a long plastic pyramid about 3/4" long falls out.

Anyone know what it's for? Structural support? Restricting ink flow?

I'm going to guess you have a cartridge fill. If this is so, then the little plastic bit you are describing goes in the puncture point that would pierce the cartridge.

I know, that makes no sense at all. Try looking at this post. If you look closely, you can just see the tip of the clear plastic part sticking out of the pierce point on the section.

Todd