Is there a way to get Spotlight to search for xattr?
Based on this, yes but you are limited to using com.apple attributes. As for extending Spotlight to any xattrib tags, you could but you will need to develop an importer for Spotlight. There are plenty of metadata tags available, I would pick one to be used as a personal tag.

If committed to searching by custom xattrib, I would give up on Spotlight and use:

find . -xattrname foo