Hello,
I did dump my Penguin Adventure cartridge and it uses a different scheme.
The way it seems to work is that it writes the bank number to $FFFD and then do a read on $C000 or $A000 (strobe chip select probably).
I did read from the connector and from the chip directly and files are the same
If anyone is interested, I can help.
I did play quickly with source code and compiled a rbf. However I'm not sure how to distinguish megacart from this one.
Thank you
Pitou!