I don't think you do. The --in param on svg-term-cli worked for me locally.

Don't need to upload. I just tested this out because I didn't want to have to upload to asciinema:

  $ asciinema rec test.cast
  <do stuff in the terminal then ctrl-d>
  $ cat test.cast | svg-term --out=test.svg

And voila, no upload needed.

edit: formatting