Hi,
I just published a guide on how to add metadata to your images in Lightroom using a donation-ware plugin: LensTagger*.
LensTagger, or: how to add metadata to images shot with legacy glass and/or on film
Using the plugin itself is quite straightforward, the only pain is having to go through several Lightroom menus every time.
But I've got a solution for that as well, at least if you're on a Mac. I wrote an Automator service that will let you invoke LensTagger with a simple shortcut of your own choice.
For the details please read the blog post (it's straightforward, but you will have to follow the instructions).
I don't have a standard Lightroom installation (i.e. my Lightroom app is not in the usual "Application" folder root), so I wasn't able to supply a ready-made file, but like I said is really a matter of copying and pasting & following my guide.
And if you like the LensTagger plugin please remember to offer the guy who developed it (Dirk Essl) at least a beer
*You can download the plugin at http://www.essl.de/wp/software/lenstagger-lightroom-plugin/