$msgtags is the built in mIRC support for handling IRCv3 tags in it’s scripting language. If it’s not available you should certainly update the mIRC client you are using.
EDIT: (include badge check instead)
ON *:TEXT:!echotest:#matt_thomas: {
if ($msgtags(badges).key == broadcaster/1) || ($msgtags(badges).key == moderator/1 )) {
msg # echotest
}
}
Inlcudes broadcaster badge assuming you want the broadcaster to have the same access.