Brennan, the Toulouse lock, is charged under European Cup disciplinary rules with allegedly entering one of the seating areas and assaulting an Ulster supporter.
Thomas is alleged to have engaged in "aggressive exchanges with spectators" and/or "aggressively attempted to enter one of the seating areas" and/or "made an offensive hand gesture towards spectators."
The panel will be chaired by Jeff Blackett, the Rugby Football Union's disciplinary chief, and will also comprise Rod McKenzie (Scotland) and Achille Reali (Italy).
If Thomas is found guilty and handed a ban it will rule him out of Wales' opening RBS 6 Nations clash with Ireland on February 4.
Brennan is alleged to have punched Ulster supporter Patrick Bamford several times, leaving the 25-year-old accountant requiring medical treatment at the ground and on his return home to London.
The former Ireland international claimed he had been provoked by derogatory chants about his mother.
Toulouse launched a robust defence of Brennan, describing the 33-year-old as a "generous man who has always respected opponents and their supporters".
"It required sustained and repeated provocation to make him react in an unacceptable but understandably human way.
"We want European Rugby Cup (ERC) to investigate the behaviour of some of the Ulster supporters," they added.
Ulster have submitted a report of the incident to ERC after receiving written statements and photographs from eye-witnesses.