To switch to OpenGFX, from the main menu click on "Game Settings" then change the base graphic set from zBase to OpenGFX.
That did the trick, the warning disappeared. I take it that OpenGFX r1016 ('LATEST') is identical to OpenGFX 0.4.7 (acquired via Bananas).
So the issue was with zBase being 'incomplete'. The zBase version used is r250 (from http://bundles.openttdcoop.org/zbuild/push/LATEST/
). Does this mean trunk can't be played with zBase or is there an appropriate zBase version elsewhere?
It means indeed that zBase has not yet been updated with that sprite. Though you can still use it to play. You'll have one, two places where a sprite will be missing in the UI. Looking funky possibly, but nothing which breaks gameplay.