The problem is not the L2 flooding.
What does a router when it receives packet for a directly connected subnet, and if the target host is not found into ARP table ? It ARPs for it.
Problem, ARP protocol is not designed to deal with multicast macs, only unicast mac. By essence, multicast uses GARP / GMRP instead.
RFC 1812 : "... A router MUST not believe any ARP reply that claims that the Link Layer address of another host or router is a broadcast or multicast address...". That's what the procurve will purposedly do, hence the need for a static ARP entry when using NLB Multicast.