Hi there,
In this topology you will be using per-flow load-balancing. Local-preference configured on the BGP routers will determine which one is used for externally received prefixes, and AS-path prepend can be used to determine which eBGP path is used for traffic entering into your network.
The device marked ???? will need to be a router with either iBGP or OSPF (redistribution occurring on your border BGP routers) configured.
Any Huawei router will be able to configure these routing protocols, the question really is how much traffic will be flowing through this device?
cheers,
Seb.