Additional information from vpxd.log also points to my hypothesis being correct.
--> (vmodl.KeyAnyValue) {
--> key = "<whateverkeyname>",
--> value = (string) [
--> "val1",
--> "val2"
--> ]
--> },
Above is what vCenter puts in vpxd.log for a VcKeyAnyValue object which has an array of strings for its value when the VcKeyAnyValue object has not been modified by vCO. That is, it is just the output from a get method which is then sent back as part of a larger object hierarchy. This works and vCenter views it as valid.
--> (vmodl.KeyAnyValue) {
--> key = "<whateverkeyname>",
--> value = (anyType) [
--> "val1",
--> "val2"
--> ]
--> }
Above is what vCenter puts in vpxd.log for the same VcKeyAnyValue object which has the same array of strings for its value if the value property of the VcKeyAnyValue's object was created as an array inside vCO. This is when vCenter rejects it as an invalid type.
Despite the value property of a VcKeyAnyValue returned from a get method being listed as having the same standard array constructor as an array created in a vCO scriptable task, the returned object has no methods typical of an array. There's no push, pop, splice, slice, or anything. Nor do attempts to force new values at appropriate numerical indices have any effect on the elements of the array. So there is no way I have found to modify the values, and no way I have found to get vCenter to accept a new supposedly equivalent and valid object in its place.