Is it just personal preference that you don't buy the locked maps anymore, or is there a specific reason?
I think the only way to get an external GPS to work with the iPhone is to use the TomTom kit; this has the advantage of powering the phone too, while airborne - there are a few apps in the store that pretend to be aeroplane GPS's that recommend using this.
Though adding it all up, the Aware GPS seems to do everything I want for not much more cash. Their website isn't crap either, so I'd have a reasonable idea of what I was buying!