As far as I know, there is no such thing as a “lossless” MPEG2 codec, as MPEG2 is a lossy compression format. I tend to either export MPEG2 directly from Symphony (when MPEG export is not broken, which it often is >:( ), or export either a Quicktime reference or uncompressed Quicktime movie which I take into Discreet Cleaner XL and encode to MPEG2 at whatever bitrate is appropriate for the job.
The highest quality MPEG2 encode you’re likely to get involves using a dedicated hardware encoder card, such as a Zapex card (obsolete), Blackmagic, etc.