If you can be sure that all non-ASCII characters fit in 8 bits (in whatever codepage that hopefully you and the receiver agree upon) the suggestions so far are fine. Otherwise I'd use this, adding a "%U" before each hex sequence to make it (better, though not perfectly unless you escape existing percent signs too) distinguishable from something like "face" or "decade" that's only valid hex digits:

perl -Mutf8 -Mstrict -Mwarnings -E '
my $x = "\x966\x959\x959\x946\x945\x961"; # Greek "foobar"
$x =~ s/([[:^ascii:]])/sprintf("%%U%x", unpack q{U}, $1)/eg;
say $x;
'
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