BTW, does this broken software run over IPv6, anyway?
Poorly designed network plus poorly designed software... I don't know which chicken came first, and it doesn't matter.
IPv6 is totally different barnyard. Build the v6 network properly -- one gateway (one router, vrrp, whatever) -- and retool the software properly. IPv6 doesn't have a broadcast address; as such if the software is setup to use an appropriate multicast target (presumably in "user defined" space), then it'll talk to exactly the right machines, and it's routable.
When I was young and the earth was still cooling, I attended my very first University level Computer and Information Science lecture. There was the normal administatrivia followed by a discussion of how lucky we were to be in the generation of programmers who would address the Year 2K problem. Just to set expectations that was 1969 (okay the earth was actually getting warmer) more than three decades before the event. Somehow I remember a bit of a scramble to get everything ready on the evening. Fast forward a few years and a bunch of us are going to see a very similar event, foretold well in advance and not addressed until the last minute. The parallels are amazing, many very large corporations will need to fix (notice the future tense) a ton of software that is not IPv6 ready and the last time any of it was reviewed was for Y2K and that guy is long since gone and it is written in a language that no one understands and testing will require at least one of each type of environment (IPv4, IPv6, Dual-Stack, ArcNet ^H^H^H^H^H^H) --Dave (How many sick days do I have left?)