I looked into this only this week.

The copy centre that scans alot of our work to send OS can do it (though I live in NSW) I'm sure there would be similiar places where you live.

They have a large flat bed scanner and can scan the backglass in one hit (the scanner) is about 90cm across for the cost of $25 per scan and they supply images back on a CD.

I'm sure 300dpi will be fine but if you can hang on I will be getting my backglass scanned Monday week and will talk to them while I'm there.