Going to be interesting to see who is released from the Wallabies squad for this one. I don't think it'll be too many given the 8 reds named but Schmidt did suggest Faessler could be to get game time. Unfortunately Hooker is one of the stronger positions in the squad depth wise, so it doesn't move the needle as far as others would.
Beyond that, I'd think that maybe Nonggorr, Lynagh and possibly one of Daugunu or Paisami might miss the 23 against Fiji and be available, I think any of these guys could come in for at least bench roles for the Reds on shortened prep, but it does require Schmidt to make a relatively early call on his 23.
All of this working under the assumption that Schmidt will be aiming to name something close to full strength against Fiji, if he wants to go a bit more experimental it's a harder to predict whose in or out.