To confirm the situation on 3.x: a unicode string with non-ascii-encodable characters is fine. The easy test here in the uk is a pound sign:

<code>
import subprocess

FILENAME = "abc£.bat"
FILENAME.encode ("ascii")
#
# UnicodeEncodeError
#
with open (FILENAME, "w") as f:
  f.write ("echo hello\n")

subprocess.call ([FILENAME])
#
# "hello" output as expected
#

</code>

So no action for 3.x. I'm sympathetic (in principle) to making a change to 2.7 but I haven't looked over the "competing" patches and assessed the ins-and-outs.