They are often not really connected internally and you would end up with “ground bounce” noise problems on I/O
Using an embedded controller board as a jumper is asking for severe EMC problems as you have just turned the daughter board into a dipole radio antenna
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Okay, maybe I should bin that idea then
You got a point, don’t bin it!
There are obviously some functionality that needs more thinking due to missing consensus.
It could be other signals than ground which doesn’t require EMC considerations.
I have been in this exact situation a couple of times, wanting to take advantage of the internal connections of components.
I know my usual PCBA house would certainly be complaining if component pin numbering didn’t match those in the datasheet. Therefore my usual workaround is to assign different but slightly similar net names to each side of the bridge (like SIGNAL_a and SIGNAL_b, and then let any routing issues that might arise be solved in schematic.