I posted your query on a forum which is sometimes visited by some who are quite knowledgeable about Taliesin/Maginogion myths etc in the hopes that they will see it and can throw some light on it but have had no replies yet. (They are perhaps more likely to be focussed on the boughs with silver/bronze bells as symbols of bardic authority but one never knows.).

Rather after the fashion of some of the answers above, i.e. more imagination than information (but hopefully more focussed on your actual question) I could imagine that it may have been a symbol that he was an emissary from Avalon (Isle of Apples). I do realise this does seem highly unlikely if the reference originates from an Occitan romance. In any case Avalon does not really seem to have any links with Arthur's court but at least it could rationalise the symbolism for a modern audience if that was your aim.

If I do get any response from the other forum I'll post it here - don't give up yet.