I don't think dispensers should be an issue, as they aren't adding an upward-facing direction for fireworks.Other things I see being broke in 1.4.6. Despensers being a pain to place now...getting the same placing program as logs. And all the mods that enchant non weapons or use magic books having to do some fancy coding. To get around the new code.
I'm more worried about what all the fireworks recipes will do to NEI. I figure some changes must have been made to the crafting system to handle all the variations without having to manually account for crafting fireworks starts with differing numbers of dyes and different extra effect items; and crafting fireworks with different numbers of firework stars with different numbers of gunpowder...