How to transform the output with a custom function?

[NextMerge]

I have a function (in this case in fish) that takes some text and transforms it, echo-ing the result.

function foo --description "Does stuff"
    // some stuff...
    echo $something
    return 0
end

How do I get a custom channel to use this function to transform the final output? This was reported (and closed) as fixed in #527 but it doesn’t seem to be implemented. The output param can only use transformations that are built in to television.

The other option would be to use an action but so far I haven’t been able to get that to work either.

If I do:

[actions.foo]
description = "Do thing"
command = "foo {}"
mode = "execute"

Then try cd (tv custom-channel) it just prints out the value of the custom command then acts as if I did cd with no argument.