Yep, if we’re lucky I think all you need to support this are atomic transactions (txes that must be included in all of the involved coins’ merkle slots in the same block to be valid) and maxtimes (to prevent your counterparty from hoarding your signature on a transaction without giving you theirs). If the operator is providing you data, then you will know for certain by the transaction’s maxtime whether it will ever be included.
If the operator is withholding data, they can force you to attempt to exit the previous coin (possibly invalidly and thus losing an exit bond), but as you mention, that’s true of your scheme as well. I think this is insurmountable (as do @karl and @kfichter I think)—in any multi-coin atomic transaction scheme where multiple parties are involved, the operator can, by strategic data withholding and cooperation with one party, cause the other party to lose an exit bond. That is not the case in single-coin-transaction Plasma Cash (or single-coin-transaction Plasma MVP) with limbo exits, but I think it is probably endemic to anything that supports atomic transactions with multiple coins. And it’s a capped, relatively small cost, that can only be imposed once, by an operator whose Plasma Cash chain you’ve explicitly decided to join, so as griefing attacks go, it is extremely modest.