[XeTeX] GSUB lookup type 3?

[Will Robertson]

Hi Jonathan

Sorry to add to the pile, but I was wondering if/how the ICU supports 
features like 'nalt'
    <http://www.microsoft.com/typography/otspec/features_ko.htm#nalt>
which, according to the spec, require the selection from a list to pick 
between various alternatives?

AAT is easy, they just enumerate all the possible types. Does (or can, 
hypothetically) XeTeX's OpenType interface support anything like this?

Thanks,
Will