Thanks for the comments.
@ ceaser: Yay, now I just have to convince a programmer to share your enthusiasm. I wonder if this change could be implemented as a server-side revision, rather than require new client downloads for everyone.
@ benmachine: I don't think the animation is much of an issue - it wouldn't affect the gameplay, and besides, the existing reload animation (player lowers gun) is pretty lame and non-descript, but that doesn't seem to bother people much. But yeah, a suitable animation for shell loading would be a nice touch.
@ David: I agree that lessening the total ammo load a bit would compensate for the elimination of shell loss due to clip reloading.
@ Noob Plox: I'm not at all trying to make the shotgun more powerful, just more complicated and skillful, and I agree that coping with reloading during hectic fights is fun; I think this proposed change would make the reloading issue even MORE of a pain by taking up more of your time, and add another subtle parameter to combat: having to spend more time and effort "feeding" your gun. Maybe there are good reasons why the idea is "retarded". If so, it would be useful to know what they are.
Addendum: Now that I think of it, using right mouse button to reload this proposed shotgun would probably be necessary, since holding down R for over two seconds would be really annoying - you wouldn't be able to strafe right during that time.