AFAIK FPC compiles for Haiku too. About Lazarus, it appears in the target list (v. 1.8.2, don't know about newer v. 2.0.0). As far as QT is supported natively by the LCL it should work.
Off-topic: This reminds me that I want to test Haiku someday. I did use BeOS a long time ago for few minutes and it was pretty nice, much more that WindowsXP.