Advsplash is an NSIS plug-in which does not have to be registered.
I assume you need the DLL only temporary, so why don't you create a NSIS plug-in or a DLL that doesn't have to registered?
RegDLL uses the path set by SetOutPath. If DLL B is in $PLUGINSDIR, the directory should be set to $PLUGINSDIR. You only have to add SetOutPath before registering the DLL.