Timestamp processor can not reference event field as timezone

When trying to reference a field as source of the timezone in the timestamp processor, filebeat complains that it could not parse the value of the field (the verbatim %{[foo]}) as timezone. Neither the documentation for this processor nor the beats documentation show such a restriction. I need this functionality to work in order to implement a use case.
I can not use an ingest pipeline here. Using the script processor is also out of the question because the timestamp format there can not be specified. So I'm kind of out of options.