Sorry, never mind that, I figured it out (but please tell me if I’m wildly wrong):
use AppleScript version "2.3.2"
use scripting additions
use framework "Foundation"
-- classes and constants used
property NSMutableArray : a reference to current application's NSMutableArray
property NSDictionary : a reference to current application's NSDictionary
set theFile to (choose file with prompt "Choose file ")
set theFile to POSIX path of theFile
set theDict to (NSDictionary's dictionaryWithContentsOfFile:theFile)
set theArray to NSMutableArray's array()
my addFileIDSIn:theDict toList:theArray
set theResult to theArray as list
on addFileIDSIn:aDict toList:theArray
set theIDs to aDict's valueForKey:"fileIDs"
theArray's addObjectsFromArray:theIDs
set theSublists to aDict's valueForKey:"sublists"
if theSublists is not missing value then
repeat with aSublist in theSublists
(my addFileIDSIn:aSublist toList:theArray)
end repeat
end if
end addFileIDSIn:toList: