Hawaii support had been broken for months (no working 3D on the open-source driver, but will work under the Catalyst Linux driver) and the few open-source AMD developers weren't tasked with fixing it over not being sure why it wasn't working and having no immediate business cases for fixing the support. Fortunately, with a bug comment made tonight, it seems things might be in order.
AMD's Alex Deucher says "it's now working more or less." Besides running recent versions of other key software packages, the "magic" to making the AMD Hawaii support work includes grabbing the latest Radeon microcode files "radeon_ucode", the latest DRM-next code queued for the Linux 3.17 kernel, and one extra patch to enable Hawaii acceleration support conditionally (the condition being based upon the new firmware file being present). If those conditions are met, the Radeon R9 290/290X open-source driver support will supposedly work.
I'll try this out shortly with my R9 290 and report back with the findings and ideally some open vs. closed source driver OpenGL benchmark results. More details on the specific changes needed for getting working Hawaii support is detailed via this bug comment.