This is a very hacky, atypical implementation.
Maybe a solution is to setup a bridge with the interfaces that connect to “A” and “B” If they form a bridge, they’ll behave like a L2 switch, open to ARPs. I haven’t had the need to setup one in my work. I tried at my home PFSense just to see how it works, but it was a completely artificial scenario.
I have used Proxy ARP VIPs when a ISP gives you several blocks of public IPs over the same interface and you need those VIPs to do NAT 1:1. I haven’t had another use for them.
In fact the description from the PFSense Book is not very clear in my opinion, but describes the use I had for Proxy ARP VIPs.
I think it’s better to somehow simplify your network. I know you said there are restrictions.