[KevinH]

Actually gumbo is not plugin related at all. You can control the mending / cleaning of text to set it on file open or save or both; or manually run mend on all xhtml files or just the current one. It handles just the non-breaking space to entity conversion. All other entities are controlled by the preserve entities preference settings.

So changing it to *not* mend on save and then running a regular expression search and replace of your placeholder to a real non-breaking space should work.