One reason why nobody has requested this yet may also be that this value is probably not put into practice very often. The vast majority of calculated album gain values is anyway in the negative dB regime which avoids clipping automatically. Still, because it's part of the standard now, I think it's better to have a full set of parameters and will definitely welcome it if MM upgrades to the new standard in a future release.
For the moment, I will follow your suggestion and tag according to the old standard, adding album peak values potentially later which should be a very simple operation. All in all, I would not like to change to another program for tagging because at my usage MM's tagging has proven quite stable across several applications what I can not say about some other tagging engines I used in the past.
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