Interesting!

I'd think, in theory the ECMP router could keep track of the MTU on a per IP basis (rather than per TCP connection) based on it having received the ICM unreachable packet. And from that moment on, sending a spoofed ICMP packet back whenever one of the servers it's routing for sends a packet the router knows won't reach the host.

But even if that works, I'm by no means a network engineer, your solution of simply broadcasting the packets is probably more efficient in the real world.