I don't think you can use Shared LUN on an external drive. the FAQ at
Can a Shared LUN be used with external devices?
A Shared LUN can only be used with virtual drives within the chassis.
I use a Promise VTrack e310s with vmware. All my luns on the vtrak are shared ones, no problem here. However you have to make sure the LUNS on the vtrak are NOT called Lun0, I read somewhere on the net that this causes troubles. vmware still calls the first lun lun0, no matter what number you give on the vtrak. However I am not sure if you will be able to boot your servers.