Update:
Now included is an add on extension to allow selection of Assets when the app is compiled. The method used to do this is to write(copy) the asset file to a temporary file in the private directory of the app (there doesn't really appear to be any other way at the time of writing). The temporary file is overwritten when a new asset is loaded. You must ensure that your file naming follows the standard format, e.g. myfile.pdf
You can download the add on extension and see the usage/blocks in the first post.