If I, a foreigner in Thailand, am the defendant named in civil litigation case in Thailand, will my passport be seized or confiscated by some competent authority?

can't really answer your question but I know the last two words are, at best, questionable if not an oxymoron

Your passport will not be seized, but the court does have the right to order you detained should you attempt to leave the country. It would require a special action on the part of the plaintiff to do this though. You can then place a bond which will be forfeit if you don't return for the court action. Unlike a criminal case there will not be any attempt to prevent you from leaving the country, nor arrest warrant if you do not appear for your court date.

There also seem to be informal policies which allow some immigration checkpoints to act without a court order for unpaid hotel and hospital bills. Not sure if this is strictly legal, but at least one TV member who ran a hotel near the Phuket airport has reported being able to do this on several occasions when clients attempted to flee without paying.