I’m relying on Juce’s cross-platform KnownPluginsList, the directory scanner and associated classes, so I can’t say exactly at which point the plugin is “not seen”, but it definitely does not apper in the resulting list.
At least on the Mac. Things are a bit different here, as you can have Universal 32/64 bit binaries. Either scan that enumerates the available plugins “sees” one or the matching portion of that combined binary, depending on the build architecture of the scanning program.
IIRC, on Windows you have different DLLs per architecture, obviously also with different names. I don’t think a 64 bit Windows program can load 32 bit DLLs. Therefore I guess that an attempt at LoadLibary() returns an error that is handled by Juce.
I haven’t had the time to verify this, though.