For Proof of Stake in e.g. Ethereum, someone that owns funds can become a "validator" by sending a special type of transaction that locks up their ether into a deposit. The process of creating and agreeing to new blocks is then done through a consensus algorithm that all current validators can participate in.
Monero already has a type of transaction, called a coinbase transaction, which produces an output with an unencrypted amount. This is necessary in order to verify that miners are creating new XMR at the consensus enforced inflation rate.
A new type type of transaction could be devised that did reveal the output amount in a similar way, in order to be eligible as a validator. Let's call such outputs VEOs (validator eligibility outputs). These VEO outputs would be clearly identifiable on the blockchain because they would be the only outputs created in non-coinbase transactions that also have non-encrypted output amounts.
It'd be important that such VEOs cannot be referenced as decoys in other transactions, because it'd be important that funds can be demonstrated to be parked as VEOs and not spent. Therefore a consensus rule would need to be written that required that when VEOs are referenced in a transaction, they can only be referenced by the owner of that VEO (i.e. no ring signature would be allowed for untraceability).
When the VEO is spent in a transaction (which would unpark the funds and remove that output's validator eligibility), a regular Monero output with an encrypted amount would be created, and so this new output could then be spent in the future with Monero's regular transaction value secrecy and untraceability features.