I had "a bit of a read" in the source code of BusyBox (v1.14.3, file: archival/tar.c). And I'm pretty sure that there is no such thing as a treatment for a comment line.
I believe it would not be difficult to enhance the code, but I doubt that Robert and the team would like to go down this road. Mind you the use of 'tar' in '/usr/bin/filetool.sh' is not limited to the BusyBox applet, so if you got tar.tgz installed this GNU version would be used instead.
Finally I did a quick test using the only sensible comment character (i.e. '#'), and both BusyBox tar as well as GNU tar are complaining (with different error messages each).