I know this was a dead issue several months ago, but has any progress been made on plug-ins which enable 3DS Max 9 to produce aircraft for FSX?

No, since there are currently no plans to do so. ACES doesn't have Max9 themselves, so there's (a) no incentive for them to update the export mudule, or (:( do so even if there were an incentive, since you can't program and test a module without something to "test it with..." ;)