At 36 Williams is due to retire at the end of the season but looked set to be stranded on 99 international caps before an injury to Sam Warburton, who captained Wales to the RBS 6 Nations Grand Slam, opened the door for one last appearance on June 2.
Williams is still behind Justin Tipuric and Aaron Shingler in the pecking order but with a tour to Australia still to come this season, Howley - stepping up in place of injured coach Warren Gatland - is likely to make the most of Wales' strength in depth.
And, while the occasion will be a historic one for Williams, Howley claims the Cardiff flanker will not just be there to make up the numbers.
"Martyn has got a lot of experience and the one thing the Barbarians have is experience and that experience is going to be vital for the Welsh players against the Barbarians," said Howley.
"I'm delighted to give Martyn that opportunity to gain that 100th cap, but the one thing in life is that nothing is given out for nothing and he'll certainly prepare for that game as if it was his first cap.
"Sam hasn't played much rugby since the French game [where Wales clinched the Grand Slam in Cardiff]. Sam's captain credentials are important to the squad and we want to look after players and give new players an opportunity."