i'd think that even with expansion, if it's welded properly, that wouldnt be an issue. .... if it werent welded properly, and the fill didnt mesh with the surrrounding area, then i could see it "pop" out.
haha I meant JB welded. There has quite a bit of discussion on actually welding the slits shut and the high potential of warping the case.
I've seen the post where someone was sucessful with RTV. I wouldn't have thought of using RTV.