Is it at all possible to use `collection_timestamp` in readonly mode?

Currently, the `collection_timestamp` method fails if the timestamp for a collection is requested on a readonly node and the timestamp doesn't exist. Are there other options besides failure here? Could we maybe fill in a default `TIMESTAMP_UNAVAILABLE` last_modified (maybe `INT_MIN`) and in this way allow calling code to continue to function with a fake but innocuous timestamp that will not cause data loss later?

/cc @bqbn