I've been through all this before.

There is no accurate and reliable method for hobbyists like us to measure LAeq. While RMS measurements are a useful indication, they don't take into account subjective or "perceived" loudness.

In fact, it seems that the AC3 tracks on most commercial DVDs have been encoded with dialnorm left at the default of -27dB, so I suspect that some professionals don't fully understand it either.

I briefly played with the Replay Gain measurement in LAME (think it gives a value as an offset from -20db instead of -31db) and initially thought I was onto something, but after further testing found that results were not so reliable.

You want to measure only clean dialogue spoken at normal conversation level, so yes on a 5.1 track that's just from the centre channel. Furthermore, it is the long term average level you are trying to estimate - if there are pauses in a conversation, that can skew the readings to the low side. You could try, as I did, editing a section of dialogue to cut out all the gaps between words and carrying out measurements on that. Great fun to hear the results, but you may still find that different sections of dialogue in the film have varying levels.

The LFL Pwnage DVD I did has 3 audio tracks all with different relative levels. In the end, dialnorm for each track was set using trial-and-error. Although the actual dialogue ended up being louder on the tracks with a lower dynamic range, the final levels are set so that the louder sections of these tracks are still at a reasonably high level.