I think the answer is no, but is there anything like a Union for Kotlin outside of Kotlin native?

Not sure if you are referring to a Union style data types but Arrow has Either, Coproduct and in general support for Union style types..

Something like where I could access a variable as either, say, an int or a byte array length 4

sealed class with data classes wrapping the underlying types is the closest you will get unfortunately but there no tagged or untagged unions in kotlin