best way to add build number to output filename?

I have an as3/mobile project going, the final output being ipa files w/ the filename specified in the build cfg

I need to figure out a way to have the output be ipa files w/ the filename specified in the build cfg + build number (the build number that lives in the ios application descriptor.... the [project name] - app.xml file

best way to do this? am imagining using ant, somehow... maybe write a custom target that reads in the app descriptor, grabs the build number, finds the output.ipa file, etc...

maybe there's a better way?

thanks