I don't know how your application is structured, but if you can split sections off into separate SWFs(plain AS3 document) then you could host those section SWFs online and require the user to download them on first run.
You could use the FileReference object to download the SWFs, save them to the File.applicationStorageDirectory, and then display/load them up with the Loader object when needed from the app storage directory. You would possibly need to check to see if the files are present at every boot of the app though as a user could "Clear Data" or "Clear Cache" of the app in the Settings of their device and one or both of those could/would delete the downloaded files.
