OK, I've given up on Breville supplying a replacement part. Maybe they'll make the group head service kit available at some point in the future (seems you can get some places to provide a quote for it for $190 USD). In the mean time, I am attempting to create a printable replacement part. I've only just started the model and it's my first one. But I'm making it publicly available:https://github.com/dvhart/scad-models/blob/master/group-collar-insert-gen1.stl
I've received one example print, and to my surprise, it actually worked (print: left, original: right):
I don't know how long it will work, 3D printing materials vary considerably in heat resistance, friction, durability, deformation, etc. The idea is to iterate on the model until it's a reliably good fit, and then experiment with materials. Once I have a known good model and reasonably performant material, I'll report back with that info. Shapeways.com can print this model today for $10-$70 depending on the material in case anyone wants to help develop the model (but please understand it is still under development).