Stewart's career seemed finished three years ago when he was advised to quit because of a serious neck condition.
However, he overcome his injury problems before resuming his professional career in England with Sale Sharks.
Stewart last played for Scotland on their 2000 tour of New Zealand but the new coach has invited him to a squad gathering in Stirling.
Williams will then assess his suitability for inclusion in the forthcoming RBS 6 Nations Championships.
Williams said: "It's a fresh start for everyone as far as I'm concerned. We have other options but Barry is a guy that I've never seen play.
"With a country with small resources we need to be looking round and finding players, and the fact that he's here means that I'm keen to have a look at him.
"We have to look at the situation. Sense has to prevail because he's playing for Sale. If we want to call Barry back into the national sport then we have to deal with the injury issue, go over the past and see if it's solvable.
"But I'm not particularly concerned about it. If there is a problem, we will deal with it, and if we can't, then we can't.
"He hasn't been selected to play for his country yet, nor has he been selected to come back next week.
"That's not for any sinister reasons but because Gavin Kerr has been cleared to play and he has participated in training this week."