I would lease the /29. AT&T puts this on the LAN side of their required customer GW and uses the high address for their equipment and I can use the other 5. I already have NAT working well and I have tested proxy-arp. The issue will come if they do, in fact, require a dhcp request and then enter a static arp entry in their gw.
My current plan if dhcp is required:
Pull the first address via dhcp with normal DHCP config.
Disconnect g0/0/0, harcode the MAC to something else.
Reconnect and pull a second IP on the new MAC.
Disconnect.
Remove spoofed MAC and set up proxy arp for the second address.
If they do not lock the arp in their device, this should work. If they do, this won't work.