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To find registry entries (and optionally COM objects) for a DLL from the command line, a combination of the answers by @virgo47 and @neves worked best for me.

Find registry entries that contain the DLL name. These entries typically use GUIDs as their keys.
reg query HKLM\SOFTWARE\Classes /s /f whatever.dll

(optional) Find COM objects that have been registered for these GUIDs. (Using /s /f "{GUID}" should be faster than | findstr /i "{GUID}". And | find /i "{GUID}" appears to be a typo (but I cannot write comments on Serverfault yet).)
reg query HKCR\CLSID /s /f "{9F3DBFEE-FD77-4774-868B-65F75E7DB7C3}"