At the moment Haiku's OpenGL implementation is using Mesa, but it's purely using swrast as a software rasterizer. (It's said to be working "very well at this point" although this classic software rasterizer is much slower compared to LLVMpipe.)
Haiku's implementation provides an "OpenGL Kit" that is a shared library with the OpenGL symbols and then linked into the OpenGL library is Mesa and Gallium3D at the back-end with the Haiku kit portion providing OpenGL renderer dispatch handling.
Alexander von Gluck of the Haiku project is wondering now what can be done next to better the Haiku Mesa support and eventually move to hardware-based rendering. While comments were being sought in this mailing list thread for the Haiku developers, two days have passed and there's yet to be any responses.
Unfortunately, it's unlikely that Haiku's Mesa/OpenGL support will change dramatically anytime soon. Back in January I talked about Gallium3D for Haiku after last year Haiku developers were working on mainline Mesa support, but not too much active Haiku-Mesa work has taken place. These BeOS-inspired developered have had hopes for a new 3D stack going back to at least 2010.