I noticed that the existence of the timer key (per Add support for service timers) is missing both here and in the other snapcraft.yaml references that I could find (e.g. https://snapcraft.io/docs/snapcraft-yaml-reference). Based on trial and error - and subsequently looking at https://github.com/snapcore/snapcraft/blob/master/schema/snapcraft.json - it does appear to be fully supported currently (without passthrough)