Currently, it is not possible to create subpackages which install files with different contents under the same path. A potential fix could provide an ability to set the build root on a per-subpackage basis, as suggested in this upstream ticket: http://rpm.org/ticket/874
Note that this would conflict with the current one debuginfo package approach. The .debug files would clash. So to support this we would also need to support splitting the debuginfo into subpackage debuginfo packages.
RemovePathPostfixes in rpm 4.13.0 appears to support this: http://www.rpm.org/wiki/Releases/4.13.0 The curl package uses it, so it's support at least on the core Fedora build infrastructure. See bug 1324623 comment 18.