After some tests, it seems that we cannot do this directly, by simply callling setAllAvailableTokensAndWrappers, but we have instead to make a small upgrade to the main IdleTokenGovernance contract.
The contract was not designed to support this specific use case (ie the change of the saved cToken address associated with the COMP reward) so a method is missing for updating the cToken variable here idle-contracts/IdleTokenGovernance.sol at develop · Idle-Labs/idle-contracts · GitHub
This means that the formal proposal for the new wbtc wrapper (which btw is ready and it’s the same already audited and in use for DAI and USDT) should be postponed until the new implementation for IdleTokenGovernance is ready and a proposal to set it has been executed. These changes can be bundled with others which are in the works and should be audited in April