Yes my 3 main loupes are all lens elements. you can't set them right on the surface you are focusing on but I don't mind holding them and focusing. I also found that if I forgot my loupe while shooting a view camera on some location that I can take a rear element off an unused lens and focus with that. I do prefer a standard loupe for focusing on ground glass.
If you need a LARGE magnifier -- as opposed to POWERFUL, high diopter magnifier -- there are lots of longer zooms, such as 70-300mm and 100-400mm, etc, that have a front element/group that is easy to remove/unscrew. They are typically 67mm to 77mm wide.
I have a couple loupes that I use to peek at the ground glass (resting on film rails) when checking focus.
They came from cannibalizing the electronic viewfinder of camcorders that were not serviceable and/or of obsolete (analog) recording technology. Ooops! what did I just write about analog technology?