Hi Tim,
I've seen the round tool for gage materials in McMaster-Carr, only problem with that one is that it stops before it gets into flat bar sizes that we often use. Quickly ID'ing a piece of 5/16 or 1/4 is my main goal, with a tapeline they both measure 1/4" and difficult to tell quickly at a glance.
As for the other suggestion, I have a UT machine with straight beam transducers already and it would work, but I wouldn't trust the guys on the floor to take care of it like I do and that thing is about $10K new.
I may have to just make a few of them on an end mill, if I can't find them in production any longer.