Michael,
if possible, get the tip version of NaviServer from bitbucket. it should work with your example with tcl 8.7a1 and probably as well with tcl8.6.*. My example did not work for other reasons, the example below is supposed to work. Handling binary data in tcl is still a pain (and a moving target).
set v "☃"
set v [encoding convertfrom utf-8 [binary decode base64 [binary encode base64 [encoding convertto utf-8 $v]]]]
ns_return 200 "text/plain" $v
