Style library deployment :
Break the inheritance of the master page and duplicate the existing master page and modify .change the master page for that Page layout Or Page (bt detaching the Page layout)
WSP Builder : duplicate the existing master page and modify and deploy in the layouts.
NOTE : CHANGES TO THE MASTER IS NOT EASY, YOU HAVE DEPLOY IT EVERYTIME :)
.change the master page for that Page layout Or Page (bt detaching the Page layout)
-> Avoid Ghosting of the new master page?? ... I assume you don't want to unghost the masterpage... is the current master page deployed in layouts or style library???
if it's in layouts...then it comes from the file sytem and I suggest osue CSS classes approach if it resolves or use WSP option...Have a look at blob cache..but I don;t think it would help for master pages :))