I have an AppEngine application that currently has about 15GB of data, and it seems to me that it is impractical to use the current AppEngine bulk loader tools to back up datase
The bulkloader is probably one of the fastest way to backup/restore your data.
The problem with the AppEngine is that you have to do everything through views. So you have the restrictions that views have... the result is that a fast backup/restore still has to use the same API's as the rest of your app. So the bulkloader (possibly with a few modifications) is definately your best option here.
Perhaps though... (haven't tried it yet), you can use the new Datastore Admin to copy the data to another app. One which only you control. That way you can copy it back from the other app when needed.