I think Qantas will be chasing IASC about Virgin's allocated HNE slot. Qantas originally applied for both the slots granted to Australian airlines, but Virgin also applied and got one which was supposed to be for BNE-HND on an A330. Without those A330s and no sign that Virgin plans to resume long-haul flying or at least any time soon, I think Qantas will apply to the IASC to get the Virgin Australia slot and also lobby the Japanese authorities to push for the slot to be enforced, and then use this to launch a second daily SYD-HND frequency once demand returns. The old 747 flight reportedly used to have very high load so maybe 2023-2024 will see the need for two daily SYD-HND flights to replace that.
I doubt the Japanese will get involved. The way the HND slots have worked is that Japan has given those allocations to counties and required the local regulators to administer how they are used and by who. They have not shown an interest in getting involved in how the other countries have used them. The only conditions Japan put on them was that they were daytime slots, to be used between 6am and 10:55pm. The is different to the Qantas's existing HND slots which were nighttime slots (between 10pm and 6:55am).
The most recent notification from IASC gives Virgin and extension until 30 October 2022 to utilize them. They've been granted extensions due to border closures, however, they would have to show a compelling reason to extend this further. Australian border closures won't be a compelling reason any longer, although Japanese might. But even then, they'd be expected to use them as soon as reasonably possible or then use them.
In the original process, Qantas argued that they automatically revert to them if Virgin didn't utilize them, but IASC denied that, but I don't see any other option than them ultimately being awarded to Qantas (even if just as a placeholder) once Virgin inevitably loses them.