How can I add a type constraint to include anything serializable in a generic method?

You can’t do this totally via generic constraints, but you can do a couple things to help:

1) Put the new() constraint on the generic type (to enable the ability to deserialize and to ensure the XmlSerializer doesn’t complain about a lack of default ctor):

where T : new()

2) On the first line of your method handling the serialization (or constructor or anywhere else you don’t have to repeat it over and over), you can perform this check:

if( !typeof(T).IsSerializable && !(typeof(ISerializable).IsAssignableFrom(typeof(T)) ) )
    throw new InvalidOperationException("A serializable Type is required");

Of course, there’s still the possibility of runtime exceptions when trying to serialize a type, but this will cover the most obvious issues.