Is placing a diode in conduction path to ground OK?
If you want to protect a board against having the wrong polarity power applied, the usual way is to have one adequately rated diode at the positive DC input, and a second one to prevent the positive rail from going more than 0.7V negative:







simulate this circuit &ndash; Schematic created using CircuitLab

The advantage of this is that your 0V reference is not compromised. I've never seen the method you propose used, and it could indeed cause problems - not least that every signal input to the device will now go more negative than the ground pin, which could cause all kinds of odd behaviour, depending on the device.

The proper use of ground reference is pretty important in just about every electronic circuit.