UPDATE: Please see the comment from Patrick for a tip. Thanks!

You can use Plugin.UserData to pass data into a plugin.

For example, from outside the plugin, you can store a list in the UserData:

p = Application.Plugins("MyTestPropertyPlugin")
p.UserData = ['a', 'b', 'mpilgrim', 'z', 'example']

In the plugin callbacks, you would access the UserData like this:

def MyTestProperty_Test_OnClicked( ):

    p = Application.Plugins("MyTestPropertyPlugin")

    if p.UserData == None:
        Application.LogMessage( "UserData is empty" )
    else:
        Application.LogMessage( p.UserData )
        Application.LogMessage( p.UserData[2] )

This will work with lists, but not with dictionaries.
Softimage cannot convert Python dictionaries into a COM Variant type. If you try to store a dict in User Data:

x = Application.Plugins("MyTestPropertyPlugin")
x.UserData = {"author":"blairs", "name":"testplugin"}

You’ll get this error:

# TypeError: Objects of type 'dict' can not be converted to a COM VARIANT

I’m afraid there’s no way around that error for Python dictionaries.
This also prevents you from saving dictionaries with SetGlobal or SetGlobalObject.