Hardsubbing ASS onto MPEG4
I'm pretty sure it's impossible to hardcode subs without quality loss. 

Unless there's a new technique developed in the last 4 years, the file needs to be converted to a lossless type then the subs need to be coded frame by frame and then compress again. 

So: There is no technique that may be used to hardsub onto a compressed video without losing [more] quality.