Sorry Will... I believe the guest user gets modify permissions when Helm adds it. To me it looks like a feature that they added but really didnt put too much attention on to it.
I will create an email today, or post, and ask the question in detail on their forum. I am in Marseille airport at the moment and have a bit of time before my flight leaves.
Hopefully the upgrade path will work well. We have quite a complex set up of Helm running which now connects 7 servers (although I think another 3 are being prepared to be added)... so I need to make sure it does work and have a good backup of the V3 database just in case
