As I understand it, the BIOS image simply has space reserved for ME in the descriptor, which is what ifdtool can find. If me_cleaner or MEAnalyzer are fine with 81723002.ME, provide it and the descriptor to coreboot. I don't know about gbe so I can't comment on that.

The paths start at the root of the tree. The default ME location is "3rdparty/blobs/$(MAINBOARDDIR)/me.bin", so try putting 81723002.ME there.