I was at FRA this morning and was going to do it. However I abandoned its pursuit due to the slow passport control lines and not wanting to re-clear security there on my US-FRA-onward EU destination with a c. two hour transit time. If I knew the enrollment office's open hours and had three hours there or would have skipped showering on arrival at FRA, I'd have done it.

I didn't notice any kiosks at the airside transit passport control, thus enrollment wouldn't have helped me save any time.