So I have one of those Rock Island doublestack 1911s, with the long dustcover. However, it has that 5 section rail at the end of the dustcover, and I'll never attach a light or anything to it because it's a USPSA gun.

I am already planning on refinishing it this winter, so I got to thinking -- while I'm at it, how monumentally stupid an idea is filling in the rail with JB Weld or epoxy or something and sanding/filing it flat before duracoating? For no other reason other than cosmetics, haha. Does the dustcover region flex too much under firing?

I've used Devcon plastic/steel epoxy on a cheap G17 slide I bought used where the previous owner sanded the finish off and Dremeled some gouge into the side under the ejection port, and it has held up pretty well so far but I don't shoot that gun often compared to the ~3-4000 rounds/yr the Rock Island sees.

Why not trade it or sell it and buy one without the rail?  You think it looks bad now, filling in the rail will look plain stupid.

Why would it look stupid, do you think the epoxy will flex and crack? It isn't hard to fill it in and sand it flush and recoat so you would never be able to tell it had a rail, I just don't know if it would hold up.

I don't want to sell because I got the gun cheap and it runs well and already has all of the mods (trigger, sights, pinned grip safety, modified magwell, springs, etc) that I would then have to go and pay for and do on a new one. Way cheaper to keep this and modify it, if it would work.

put a weight up there instead?

I'm not a fan of rails on pistols either. You could always take a grinder to the rail and reshape it before its finished.

After repeated draws / reholstering, that filled area will look like hammered shit.
Remove the rail instead.
Grinder, Dremel, files, finish sand with grit matching your existing finish.
Go slowly, check straightness often.

I use a race holster for it, so it probably wouldn't get beat up.

I never got around to doing anything about it this winter though.