Earthing SELV LED driver.

That is a very curious list Graham because none of those things requires an Earth connection, and if the design does it does not properly meet the SELV requirements.  Let's actually examine what you have written:

1. EMC is not in any way affected by an Earth wire of unknown length, routing, or anything else, in fact, it may well increase EMC levels. Whilst effective RF Earth connections may reduce EMC under certain circumstances, in the generalised case they cannot, because there are far too many unknown conditions. The normal way to reduce conducted EMC is to use common-mode transformers (often called chokes) so that the signal on each live wire is equal and opposite in polarity, so that they largely cancel in the connecting cables, therefore radiate very little. Capacitors between live conductors may be effective for some frequencies but tend to make the signal on both live conductors equal, which cancels out the differential effectiveness of method 1.
2. As the objects are in plastic cases and the output is completely isolated, what might need Earthing as nothing is accessible?
3. Why does a static charge on the plastic matter, it will be tiny?
4. Surge protection inside the equipment only needs to deal with voltages between the live conductors unless Earthed!

That is a very curious list Graham because none of those things requires an Earth connection, and if the design does it does not properly meet the SELV requirements.  Let's actually examine what you have written:

1. EMC is not in any way affected by an Earth wire of unknown length, routing, or anything else, in fact, it may well increase EMC levels. Whilst effective RF Earth connections may reduce EMC under certain circumstances, in the generalised case they cannot, because there are far too many unknown conditions. The normal way to reduce conducted EMC is to use common-mode transformers (often called chokes) so that the signal on each live wire is equal and opposite in polarity, so that they largely cancel in the connecting cables, therefore radiate very little. Capacitors between live conductors may be effective for some frequencies but tend to make the signal on both live conductors equal, which cancels out the differential effectiveness of method 1.
2. As the objects are in plastic cases and the output is completely isolated, what might need Earthing as nothing is accessible?
3. Why does a static charge on the plastic matter, it will be tiny?
4. Surge protection inside the equipment only needs to deal with voltages between the live conductors unless Earthed!