I thought about that possibility, but it is so clearly Miranda Richardson, that, from a filmmaking standpoint, it doesn't seem to make much sense.
The script also makes a point of having the auctioneer address her as Madame Giry in the opening sequence. While unmarried older women might be addressed as "Madame," this might be asking too much of the audience to understand, without thinking about it in great detail later, that this is Meg.
Also, Meg deliberately leaves the monkey music box and takes the mask from the Phantom's lair. It is Madame Giry who would understand the music box, having seen the Phantom with the one he made as a boy when she rescued him. That would give meaning to Madame Giry bidding for it.
Therefore, I would disagree that it is Meg.
Whew! That was way too much thought for a film I didn't really enjoy!
"I am open, and I am willing,
For to be hopeless would seem so strange.
It dishonors those who go before us,
So lift me up to the light of change."
Holly Near