New answers tagged compatibility
(Edit: after re-reading your question I realized I misinterpreted the stud with the bar. Leaving the answer for posterity.) There is. The size in the small holes beneath newer plates is the same size as a connector that doesn't have a fixed name. Bricklink calls it pin, others call it mini pin or stub. It is used in minifig headgears and a few other places, ...
The LEGO Scala flowers and butterflies also fit. You can see them on this page: http://www.bricklink.com/catalogList.asp?catType=P&catString=102 Also the LEGO Belville, flower, butterfly (same as Scala) and ladybug. You can see those here: http://www.bricklink.com/catalogList.asp?catType=P&catString=48 And from LEGO Friends all the other flowers ...
For base plates you can connect the light sabre pieces underneath the base plate but I don't think it was an intentional feature
If I'm understanding you correctly, you're talking about the molding cavity directly underneath the stud (that actually goes up into the stud), and not the main tube cavities under plates/bricks (that are actually offset from the studs). If that's the case, no, I'm not aware of any element that "intentionally" fits that space. First off, I don't think that ...
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