With the match falling outside the IRB Test window, non-Welsh clubs aren't obliged to release players and Hook's new employers Perpignan have ruled he will be required for their match against Lyon.
Lydiate returned to action for the Dragons in the Amlin Challenge Cup victory over Cavalieri Prato but the flanker suffered a recurrence of an ankle problem he experienced during Wales' World Cup campaign.
Lock Charteris, who also significantly enhanced his reputation over the course of the World Cup, is still in plaster with a wrist injury and won't find out the extent of the damage until next week.
Both however should be fit for the start of the RBS 6 Nations but Charteris admitted his frustration at being kept under wraps ahead of the rematch of the third-fourth place play-off.
He said: "At this moment, my wrist is in a cast and I don't know what it's like under it.
"I just have to wait and see but, hopefully, I will be back for the game with the Scarlets.
"It's just frustrating coming off the back of the World Cup because I want to get back involved and play for the Dragons."
Lydiate is rated even more doubtful after being on the receiving end of a big hit in the 33-3 victory over the Italians.
Doubtless Gatland will want to field his first choice back row of Lydiate, Sam Warburton and Toby Faletau against the Wallabies after Warburton missed the third-fourth place play-off through suspension.
But Dragons coach Darren Edwards is adamant the 23-year-old will not be rushed back into action before he is ready to return.
"He was obviously not fully recovered from the previous injury," Edwards said. "It's important we make sure he gets it right.
"It's the same ankle but he's able to walk around without support. He's in trainers, not a space boot.
"Dan's desperate to get back on the field and play again but it's like any injury, you don't fully recover from it unless you have enough time off.
"The tackle which injured him was pretty ferocious and Dan was fortunate he didn't break his leg."