Well, the tag types allowed by the Designer are listed here: Tag Data Types - Ignition User Manual 8.1 - Ignition Documentation
Right now the whitelist in the code is this:
public static final Set<Class<?>> STORABLE_TYPES = Set.of(
Date.class,
String.class,
Integer.class,
Long.class,
Short.class,
Float.class,
Double.class,
Boolean.class,
Color.class
);