Maybe a load of nonsense?
If you are going to Sabotage this weekend I have a din fill rig you could use for the day. Don't know about the converter you need though.
My patchy understanding that as long as the din fill rig is rated to the bar of the tank then the fill rig should go straight on. I think most din fill rigs are rated at about 232bar to match a 232bar (3000psi) dive tank. I think you may need a different bit of kit to fill from a 300bar (4500psi?) tank, such as an A-Clamp fill rig.
Anyhow.... I'll bring my fill rig to Sab on Sun and if it helps then I'll come begging for top ups from you.
Tom Allen on the forum is the expert regarding air and fills, and will be able to give you techy stuff and also help you out with any bits you need.