A little late but nonethless, the Haiku additions should now be part of the VirtualBox tree. The base PCI VMMDev driver along with mouse integration has been tested personally by me and known to work. Currently there is no binary packaging for the additions that allows for easy installation. This will be done as follow up work along with fixing bugs and general cleanup.
The patch for adding a guest OS type for Haiku is outdated as there is no longer as XSD for the guest additions. Feel free to submit an up-to-date patch for the Haiku OS guest type.