there is some chan/port that always return a value, but can be closed?

I think that I can reify impl/UnblockingBuffer impl/Buffer

this might not be what you are looking for, but a promise-chan always returns the first value it received, even after closed

or you could just create a go loop that constantly adds the same value to a channel

Something like the duplex channel from kinsky could be of inspiration: https://github.com/pyr/kinsky/blob/master/src/kinsky/async.clj#L10

Use the up channel to send a close instruction

or just reimplement the protocols as you need