Due to some legacy design choices, it is hard for Teiid to a column with '.' in it regardless of the quoting. As you say any other character is fine.
The workaround would be to not use the header and to instead positionally specify all of the text columns.
We should find a solution for this (it came up recently for [TEIID-3073] Tableau Connection Issues - JBoss Issue Tracker as well) or at the very least address it specifically for TEXTTABLE, can you log an issue?