What type of citation style are you using, i.e. author name and year or numbers? If you are using numbered citations then you only need to make changes to the actual bibliography listing and the following method should work:
- Wait until the very end once you are sure that you don't need to run BibTeX again, i.e. when there are no further modifications to the .bib file needed and all the citations are in place in the .tex file.
- Do the usual LaTeX->BibTeX->LaTeX dance one last time just to be sure.
- Open the .bbl file in an editor and do a search and replace on "et al."
Now, next time you run LaTeX your bibliography will have your replacements in it. Be aware that running BibTeX again by accident will overwrite the .bbl file, so you might want to keep a copy of it in another directory just in case. Alternatively, you can just remove the bibliography commands from your .tex file and copy and paste the contents of the .bbl file in its place to have an old fashioned non-BibTeX bibliography.
BTW, editing the .bbl file like this can be a good way to correct minor annoyances in the way BibTeX works without writing your own stylesheet, but if you find yourself doing it a lot then it is probably time to bite the bullet and write one.
If you are using author and date style citations then I don't know how to change the "et al."s appearing in the main document itself without going in to the style file.