Yes, there are occasionally contracts paid out by the Inc. to developers to work on things, but this depends on a developer having the opportunity and willingness to do so, since the funds will eventually dry up and they will be out of a job.
We had two major contracts (for a few months) in 2012-2013 to finish and then integrate the package management system, another in 2013 for the new kernel scheduler, and then one that lasted almost a year around the same time working on the WebKit port used in the built-in web browser (which ended because the money ran out.)
Haiku, Inc. currently has over $100k in the bank as per it's last financial report, but at only $20k (usually less) in donations a year, it's obviously not enough to be sustainable past a year or two for even a single developer. If there was actual sponsorship by the community or otherwise, then yes, a number of us would be willing to work on Haiku full-time.