Could NSIS get clsid of a dll file?

probably yes.

most of the CLSIDs on my system have a subkey named "inprocserver32". in this subkey, the default value is the name of the dll file.

so all you need to do is to loop through the whole clsids, which will probably take a long time, and read all the dll names and compare them with the one you need. then you can extract the clsid out of the currently read registry key name.