You don’t need to wait for the OS to be finished. But, don’t count on the same developers working on both the OS and office suite (and web browser and …).
Even though an office suite is a complex product, it is a little simpler than an operating system. So it should be possible to gather a team of people to start working on it. I would love to see more 3rd-party developers on Haiku.
Regarding Abiword, even if the GUI can’t be reused directly (for stability problems), at least they had the nice idea to split out the file format management into separate libraries. This is at least one thing that don’t need to be re-done. Maybe these libraries could be converted into translators for Haiku, possibly allowing conversion from docx to pdf as a starting point? A small first step, but being able to do that would allow me to reboot to LibreOffice on a non-Haiku in many occasions.