This is also what VTT does when one does "Prepare font..." and a character is not present in the character groups template file ("CharGrp.txt"): it gets assigned to the default "AnyGroup" group, or 0.
Note that the TSI1 and TSI3 tables do not raise when a glyph name is in the glyphOrder but is missing in their internal mapping; they instead write an empty entry.
This makes easier to merge TSI* tables from different fonts or from earlier revision of the same font containing only a subset of the glyphs.