Hmmm. Interesting. This looks like a quirk in the GUI. When I first try to create a new property, trying to select BYTE (and a few other types) change the cursor to a "prohibited" symbol. If I then select some type that is "allowed", I can then go back (before saving) and select BYTE. At that point, a property with type BYTE is successfully created. But I do also notice that when querying records in this class, the GUI shows the BYTE property as [binary-data] in the table display (in the raw display, it's just a numeric value, as expected).
By the way, what version of ODB and what browser are you using?