VASA 1.0 is supported by CoprHD. VASA 2.0 (which included SPBM and Virtual Volumes) is currently under development and hence not supported.
VAIO and VAAI are data path APIs, and if the array supports them, CoprHD will not come in between (in other words, the volumes and filesystems will show up as 'hardware accelerated' and vSphere will use these APIs to speed up certain operations).
Multipathing is supported by CoprRD. You can specify paths policies in a virtual pool. You will need multipathing software installed on the host.
Storage DRS is purely vSphere side feature and so should work fine with CoprHD.
Your idea of writing a Southbound driver for your storage for CoprHD and thereby supporting VMware and OpenStack makes sense.
Regards
Anil