Tigers chief executive Brendon Gale told Fairfax Media his club would seriously consider blocking a move of such a valued member from their football department so late in the year.
But with the Saints making the stunning decision to sack senior coach Scott Watters last week, Williams has been thrust into favouritism to take on the top job at Seaford.
MORE: Who is the next St Kilda coach? | Scott Watters: Another St Kilda debacle
And AFLCA boss Danny Frawley said Richmond would have no right to prevent the former Port Adelaide premiership coach from being Watters' replacement if St Kilda intended to go down that path.
"They (Richmond) haven't got a case," Frawley told the official AFL website.
"There's nothing they can do if he's offered the senior role at St Kilda.
"They could offer him a longer-term deal, but it's very hard to stop an assistant becoming a senior coach.
"That's a job promotion, so there's not a lot the Tigers can do, I wouldn't have thought."
Williams crossed to Richmond for the 2013 season after being denied the chance to succeed Kevin Sheedy as senior coach of Greater Western Sydney, where he spent two years as an assistant.
Leon Cameron was instead anointed as Sheedy's successor and has begun coaching the Giants in his own right following the four-time Essendon premiership coach's retirement at the end of 2013.
Williams is considered to be one of the best coaches of the modern era, having guided the Power to their one and only flag in 2004 during a tenure that lasted 12 seasons and included 257 matches.
The 55-year-old oversaw Port Adelaide's most successful era which saw them win their premiership, claim three consecutive minor premierships (2002-2004) and play seven finals series in nine years which also included three preliminary finals in a row.