I thought about writing a shell script that would modify the CFBundleDocumentTypes arrays in Info.plist files. But each app has multiple keys (sometimes an icon) that would need to be changed.
lsregister can't be used to make specific modifications to the Launch Services database.
$ `locate lsregister` -h lsregister: [OPTIONS] [ <path>... ] [ -apps <domain>[,domain]... ] [ -libs <domain>[,domain]... ] [ -all <domain>[,domain]... ] Paths are searched for applications to register with the Launch Service database. Valid domains are "system", "local", "network" and "user". Domains can also be specified using only the first letter. -kill Reset the Launch Services database before doing anything else -seed If database isn't seeded, scan default locations for applications and libraries to register -lint Print information about plist errors while registering bundles -convert Register apps found in older LS database files -lazy n Sleep for n seconds before registering/scanning -r Recursive directory scan, do not recurse into packages or invisible directories -R Recursive directory scan, descending into packages and invisible directories -f force-update registration even if mod date is unchanged -u unregister instead of register -v Display progress information -dump Display full database contents after registration -h Display this help