Set AC3 Surround flag without re-encoding

[Sudum Gumudu]

I've noticed that quite a few titles with 2.0 DD actually contains Prologic coded sound, including the project I'm working on now. I would like my amp to autoswitch to Prologic decoding when playing it, but as the DD track is encoded without the Surround flag, I have to manually switch the amp. This is fairly common, and somewhat irritating.
Is there a way to process a DD file to set the flag without re-encoding, and lose quality? I would love that. I can't be the only one bugged by this.

[ursamtl]

It may be that in cases where the flag isn't set, the file was not encoded with extra surround info for Dolby Pro Logic decoding. By nature, true stereo recordings have ambience information present in the "difference" signal (what you get when you subtract one stereo channel from another electrically). Pro Logic decoders extract this ambience and put it in the surround channels, even if the proper encoding was not carried out on the source. It's a good bet that there was in fact encoding carried out when the flag is set for a 2-ch DD source.

[Sudum Gumudu]

Well, I made that determination empirically, by switching the amp to Prologic mode, and listening to the back speakers. On the menu sound, they are quiet, and in the movie they are not. Also, I disconnected all front speakers, and the dialogue was barely audible. The dialogue is markedly more prominent in the center speaker.
Is there a way to detect the presence of Prologic coding objectively?

[ursamtl]

Well, I made that determination empirically, by switching the amp to Prologic mode, and listening to the back speakers. On the menu sound, they are quiet, and in the movie they are not. Also, I disconnected all front speakers, and the dialogue was barely audible. The dialogue is markedly more prominent in the center speaker.
Is there a way to detect the presence of Prologic coding objectively?
The only way I know of is the flag. By the way, as a rule dialog is mixed to the front center, so even if the material isn't encoded using Dolby techniques, chances are you won't hear it in the surround channels when regular stereo material is played back through a Pro Logic decoder.

[FlimsyFeet]

This thread's been dead a month but from doing a search it's the closest thing I could find. However there's no answer at the end of it!

So - does anyone know if it's possible to set the surround flag in an AC3 stream without re-encoding?