WillLem sent an e-mail to Volker Oth, the author of Lemmini, asking for permission for SuperLemmini to include the Windows Lemmings assets. Volker replied, and WillLem sent me copies of his and Volker's messages.
From what I can tell, Volker forbids the inclusion of Windows Lemmings resources only to (possibly) stay out of legal trouble, and doesn't intend to actually enforce that policy. Also, he didn't seem to care what I do with SuperLemmini, at least as far as Lemmings assets are concerned.
So, I now announce that, starting with the next version of SuperLemmini, extraction from the Windows version of Lemmings will no longer be required; instead, all the required assets will be included, ready to use. This will address any problems related to the extraction process, and should make SuperLemmini a bit more accessible (since it won't require a version of a game that's probably long out of print).