TG won't even employ female pilots so expats are probably never going to happen (again). BKP did employ a few Air Berlin FOs on contract last year and there is at least one European expat ATR Capt.
The other major Thai carriers seem to be able to still satisfy their requirements from within so there appears to be no need (yet)
As safelife states, there are quite a number of expats legally based here for non Thai operators, both European and Chinese as well as a whole raft of others on a range of other operations.
To making sweeping statements that "Thailand is only for Thai " and "they won't give visas" is factually incorrect.