is there something like jsvArrayBufferIteratorGetByteValue ?

Use jsvArrayBufferIteratorGetIntegerValue - the Byte write was (I believe) a special case because the writing of a byte is easier - but for reading there wasn't as much of a difference so I left something more general purpose. You can still do:

jsvArrayBufferIteratorSetByteValue(&itDe­stination, (char)jsvArrayBufferIteratorGetIntegerVa­lue(&itSource))