I think changing the appearance of the undead would be your best bet. You mentioned that the only real issue is that the party will be able to spot them easily, which is what results in their death. It'd therefore make the most sense to just solve the "party spotting the undead" problem. Making them look like something else, or making them invisible, are both solid options.
The first on-topic reply.
Seriously, how many times does the OP have to explicitly state that OOC solutions are neither helpful nor wanted before people stop using his thread as their personal garbage dump? If you can't think of any solutions to his problem (and he's stated multiple times in multiple posts that he just wants in-game suggestions), that's cool; there's plenty of problems in the world I'm not creative or knowledgeable enough to solve within their required parameters.
The difference is that you need to be able to recognize that fact, and not try to force your own playstyle and group dynamics onto other people who've explicitly asked you to stop. Please have some respect for the creator of the thread.