I understand this is probably quite late answer, but since I found you article by googling, and then found a solution to my problem (which is similar to yours) I decided to add information for the others.
The UI of the Oracle VM Manager restrict the MAC addresses that you can enter.
It turns out that if you import a VM machine (not a VM template) it allows you to keep the original MAC address. The same is true for P2V VM creation method.
If you have followed a support.oracle.com note about VM guest migration from OVM21 to OVM30 that you probably imported a VM template in OVM3, and then created a new VM, with one of the preconfigured VNICs. Actually you can create a new VNIC with whatever MAC address you want from the CLI (command line interface).
ssh admin@you-vm-manager-host -p 10000
create Vnic name=desired-mac-address network=one-of-your-defined-networks
Then you can edit you VM machine configuration and choose this address, instead of the one assigned to your VNIC.