I haven't found many programs that let you modify the lens name in EXIF. Lightroom is like the rest. If I set SR correctly, I can find photos taken with my 400mm prime but the name of the lens is "A Series Lens".
Exiftool is a great utility but something running within LR would be ideal.
LensTagger looks like it works so far. It uses Exiftool to do the work, but runs as a plugin within LR. I just changed the name of a few lenses and am happy. The site got me started with setting up the plug-in. I had trouble getting the exiftool.exe file in a place where the plugin could find it, eventually just putting it in the Windows directory.
I am just starting to get organized with LR4 so I did not worry about file modifications or sidecars or any of that. If I had every edit and keyword for years in there, I'd be more cautious.