I think the 7615 is your easiest option from a legal standpoint, but it would involve some gunsmithing (meaning $$$). It would have to be rebarrelled for 7.62x39, and the bolt face opened slightly. If they are already acceptable to your gruppenfuhrers as you say, then they're unlikely to raise objections to a mere caliber change. Best to avoid things like pistol grips, however, that can trigger knee-jerk alarm bells in the wrong places. You could have one rifle shooting both 5.56 and .300BLK by swapping barrels, but I don't know how difficult it is to swap barrels on the 7615. You couldn't have one rifle that could be used for all 3 cartridges, since the 7.62 bolt face is different.

I'd like to say that a 7.62x39 version of the 7615 could be a major seller in Oz and other places that ban semi-autos (Canada appears likely to join that list at some point, if our current administration has its' way). Unfortunately, if they do become widespread, it's only a matter of time before Big Brother realizes how fast they can be and bans them too. It only takes one looney using one for the wrong purpose to ruin it for everybody, so quantity production is out. I think the same would apply to any centerfire pump rifle. However the occasional conversion wouldn't attract much scrutiny.