IPv5 = RFC1819 Internet Stream Protocol
EUI-64 is just a way to extend a MAC-48 to give a unique IPv6 address (adding FF:FE between OUI and Device Identifier)
The MAC address 0021.86b5.6e10 (48 bit) becomes
the EUI-64 address 0221.86ff.feb5.6e10 (64 bit)
if you employ the same space saving measures used in IPv6 with the smaller address space and extend the ASN field from IPv4 you end up with plenty addresses to use!
2^48 is more than enough address space, and when your talking 4 addresses in an IPSEC packet, it makes a huge overhead difference.