I'm afraid that one is a limitation that only Chaos can resolve. We use the same underlying techniques as V-Ray Instancer so if something doesn't work with Instancer it won't work with Forest Pack or RailClone either. If it doesn't work with Instancer, it also means we can't work around it without Chaos' help.
One workaround that will help is to instantiate it with RailClone tools.
I hope that makes sense