I'm don't think there is anyway to invert the thumbnails used by Face Recognition, but there is a better option.
Why not use Export or 'Edit in PS' to convert the camera raw images to 16 bit ProPhoto RGB TIFF. Next invert the image files in PS and convert them to Grayscale mode. You can use Batch processing to apply these steps for a group of image files. Then save the image files and process them inside LR. The file size is not outrageous since it is Grayscale and you can apply ZIP TIFF compression to reduce it further. You can also apply special processing unavailable in LR using PS (Content Aware Fill,etc.) to the "base" TIFF file.
This article may be of interest concerning processing color negative images. Many of the techniques used apply to B&W negative processing as well.
A PDF with full workflow-details is available in the above article.
Thanks for your advice and for the link to Luminous-Lanscape. I know this place and often go there to get informations.
I downloaded the PDF file: it seems very interresting. I'll try to read that in details tomorrow (French time... )..
And I'll try the process with few pictures to get an idea of the results, and try to know how long It'll take for my thousands of pictures I have to work on!