When we eventually come to work on this area some more, including handling the appropriate metric modulations to define the relationship between the irrational and surrounding rational meters, we may be able to handle the drawing of incomplete tuplets as part of that work, though it’s not yet scheduled.
There aren’t any glyphs in Bravura for drawing tuplet brackets that would be any use: Dorico draws them as primitives. You can’t adjust the hook length of each end of the tuplet bracket independently, so unfortunately you can’t use a real tuplet.
I don’t think there will be a good way of doing this that isn’t pretty labour-intensive, I’m afraid.