You could also consider XMedia-Recode which is what I use but I don't have a 5.1 source to hand to double check but I'm sure I recall DPLII downmix option. I expect regardless of your method you will need to demux so I'd suggest MKV container and then use MKVToolNix to extract your audio stream. Both XMedia-Recode and MKVToolNix are available in portable format. Unless Handbrake and/or XMedia-Recode can transcode to your final lossy codec then perhaps downmix to FLAC as an intermediate and convert from there in Foobar.
If you're familiar with AVISynth then the follow function should work. Note that LFE is dropped.
Function DownmixDPLII(Clip AudioStream)
{
Front = GetChannel(AudioStream, 1, 2)
Center = GetChannel(AudioStream, 3, 3)
RearLeft = GetChannel(AudioStream, 5)
RearRight = GetChannel(AudioStream, 6)
SurroundLeft = MixAudio(RearLeft, RearRight, -0.2818, -0.1627)
SurroundRight = MixAudio(RearLeft, RearRight, 0.1627, 0.2818)
SurroundMixed = MergeChannels(SurroundLeft, SurroundRight)
FrontCenter = MixAudio(Front, Center, 0.3254, 0.2301)
Return MixAudio(FrontCenter, SurroundMixed, 1.0, 1.0)
}
Actually, EAC3To also has an DPLII downmix option so that would be worth testing as well.