Your best bet is to contact Symantec Support as even if someone from Symantec answers this question on this forum and says it is supported you will have a hard time logging a support call, because one of the first things Support check is the HCL and if hardware is not listed, they will ask you to comply with HCL, so you will need a Support Exception for this to be supported of for the HCL to be updated. The current documentaion is not very clear, so it may be supported - in particular, for third party multipathing with VCS wth LVM (not VxVM), link http://www.symantec.com/business/support/index?page=content&id=TECH146404 specifically for RHEL says
If SAN LUN multipathing is a requirement for the VCS cluster, the recommedation is to migrate LVM data to VxVM 5.1 with its bundled DMP multipathing or use other VxVM compatible third party (TPD) multipathing software (e.g. EMC PowerPath).
So this implies EMC PowerPath is supported with VCS and LVM, but this is NOT listed by the 5.1 HCL and this only lists VCS and LVM with third party multipathing for AIX:
The VCS LVM agent supports the EMC PowerPath third-party driver on EMC's Symmetrix 8000 and DMX series arrays.
The VCS LVM agent supports the IBM MPIO third-party driver on IBM's DS8000 arrays.
The VCS LVM agent supports the IBM SDD third-party driver on IBM's ESS750/800 and DS8000 arrays.
The VCS LVM agent supports the HITACHI HDLM third-party driver on Hitachi USP/NSC/USPV/USPVM, 9900V series arrays.
Note Powerpath is mentioned for most platforms including AIX and RHEL for specfic arrays so I would assume with is the VxVM, not VCS with LVM, but this is not 100% clear. Assuming this is VxVM, then this means Powerpath is supported with Linux, but
http://www.symantec.com/business/support/index?page=content&id=TECH51507 gives "Guidelines on using OS Native Multi-pathing with DMP" but this is only for AIX and Solairs, so this really needs updating to mention LInux and HP-ux.
The settings shown for arrays reflect those used in Symantec’s testing, and are not necessarily the only supported settings.
So I would log a call with Symantec to ask for clarification of support for RDAC with VCS for both LVM and VxVM.
Mike