What stops you from changing the 802.1ad bridge port pvids to unique
values, like 3, 4, 5... instead of 3, 3, 3, and making each other
j != i bridge port be a non-pvid member of port i's pvid?

I'm not sure if I understand this suggestion.

I tried to draw out what you described here:
https://docs.google.com/drawings/d/1UcOpENFgr-s6p8Ypwo-l4yTvtUZFM6vSLxLiX2FOMLU

I'm not sure how host A can communicate with B with this configuration.

Consider host A transmitting towards host B. When the frame leaves
".1q bridge 3",
it will be tagged with .1q tag vid 7. When the frame leaves the .1ad bridge
heading toward ".1q bridge 2", it will be tagged again with an outer
.1ad tag vid 3.

So ".1q bridge 2" will see the frame as having an outer tag of .1ad vid 3 and
inner tag of .1q vid 7.

Is that what you are thinking, or something else?