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Leaving my notes here for something that didn't work. Hopefully this will save other people some trouble...
Given I can't find two-part silicone locally I decided to try Permatex "Ultra Copper" maximum temperature gasket maker. This is a silicone sealant intended for making engine seals and gaskets and is rated to 700F / 371C.
I used mineral turpentine to thin it down to a runny custard-like consistency and trickled it into the mold with a toothpick. When I demolded I discovered that it still wasn't runny enough to make it to the bottom of the mold and, as expected, it shrank as it set - the turps evaporates off so it loses some volume.
I'm going to have to source some two-part silicone from Europe (Germany) by the looks as the ones I found in my country are only rated to 200C and I need it to print PET-G at around 240-250C.
If someone has a known-working brand for this I've love a PM with the info, thanks!