Hi,
I had very similar question once and the answer I've got from Phil was:
"...and I assume you want to specify a tag by number to distingush between the 2 lenstype tags? The only way to do this is to create a user-defined tag to override the exiftool definition. You can't specify a tag by number." (see: http://www.cpanforum.com/threads/7987 )

Greetings,
Bogdan