I like to use bullhorns for commuting but have been struggling with how to reconcile these with an efficient fold on the Helix.
Perhaps fitting some kind of quick release mechanism to the stem face bolts to allow easy rotation of bull bars down to align with the steerer tube before folding. This would minimally impact the folded size. There doesn't seem to be anything commercially available and any such solution would have to be jerry rigged. The Tern Andros stem might work with a T adaptor for the steerer.
Otherwise, Billibars could be the answer if they become commercially available. Would need to find a way to stow the removed bars on the bike though.
Adjustable angle riser stems are limited (max 60 deg each way) which is probably not sufficient.
Thoughts appreciated if anyone has experience of tidying away bullhorns on a folder.