Salter left the club to play rugby union for West Hartlepool at the end of last season, but London retained his registration, enabling him to play in the Cup immediately.
With their two senior props, Darren Bradstreet and Grant Young, both injured, Salter will be on the bench at Headingley. The Broncos coach, Dan Stains, has stepped up his attempts to enlist reinforcements since Young broke his leg at Sheffield on Saturday.
"I'm very pleased to be able to carry on the work that the Broncos had done with Matt and I'm looking forward to seeing him in action," Stains said.
Salter, aged 22, and originally from Blackheath, left London for a more lucrative offer from West Hartlepool last October, but that club's financial problems have left him looking elsewhere. He has signed with the Broncos for the rest of this season and will then assess his options.
It is a sign of how badly stretched London are that they should be including a player their coach has not even seen for a game of this importance, but Salter will act as vital back-up for two second rowers, Steele Retchless and Shane Millard, who will be used at prop.
Their opponents coach, Castleford's Stuart Raper, does not believe that they will be weakened by the pair's presence in the front row.
"I'm a big fan of blokes like that," he said. "They're not very big, but they go forward all day and can often be hard to tackle."
Raper is confident that two of his three casualties, Dean Sampson and Aaron Raper, will be fit for the semi-final. There are still doubts about Danny Orr, whose chances Raper describes as no better than even.
"He's more confident than I am, but I'm told he's a quick healer," he said of his stand-off's knee injury.
Salford have launched an internal disciplinary inquiry into Andy Gregory's latest threat to resign. Super League's longest-serving coach hinted strongly that he would leave after his side's defeat at Wakefield on Sunday, but withdrew the threat on Monday.
Yesterday, however, the club's board of directors said that they were "disappointed" by his remarks after the match.
"His comments were not in the best interests of the club and its supporters," said a statement from Salford. Gregory has been in similar trouble with the board before, although there is unlikely to be any immediate question of sacking him, partly because the club's chairman, John Wilkinson, is away until the end of the week.
Patience could have worn thin with Gregory's outspoken behaviour, but sacking him could be an expensive exercise and, despite crying wolf on several occasions, he shows no sign of going of his own volition.
The Halifax utility player, Martin Moana, who missed part of last season with a broken arm, has suffered a similar injury in the victory over Huddersfield at the weekend.
The St Helens forward, Paul Davidson, has been suspended for two matches for the reckless use of his forearm in the match against Gateshead on Sunday.Reuse content