1. Edit a local copy of cont-au.bst and insert a function that will make the author name uppercase. bst is infamous for its incomprehensible syntax, but it appears to be possible. this would be the proper way. google is your friend here.

Yes, very incomprehensible. And another thing, I couldn't find any "authoryear" word inside bst files. I think it usefulness is only for publications list.

2. ConTeXt produces a .bbl file from which it retrieves the bibliographical information. The authorname used in the \cite command is captured in a key=value of the form
a={{Hagen}},
Write a simple lua script which will make the captured name uppercase and write everything out to a new file. Then, in your preamble, replace the \setupbibtex[database=...] line with
\usepublications[<name>.bbl]
So this is something you would do at the very end of the editing process, when you're sure that all the references are in the bbl file.

Thank you. It's the best solution to my problem. But I'll do a Python or Bash script to do this, as I'm more used to those languages, and I'll add as a target on my Makefile.

It may also be possible to hook a lua function into the process when ConTeXt typesets the value from the bbl, but that's beyond my knowledge, Hans is the only one who knows the internals of the mkiv bib mechanism well enough to say if/how this may work.