I don't understand why you want to add a custom download tool? Apt already supports a variety of protocols. If you want to support new protocolls you should target apt itself. This way all tools which are based on apt could make use of it transparently.
There is already a branch of mine in which I started separating the download bits from the cache modifying and the install ones:
Hello James, I am the main author of aptdaemon.
I don't understand why you want to add a custom download tool? Apt already supports a variety of protocols. If you want to support new protocolls you should target apt itself. This way all tools which are based on apt could make use of it transparently.
There is already a branch of mine in which I started separating the download bits from the cache modifying and the install ones:
lp:~aptdaemon-developers/aptdaemon/separation
Aptdaemon in this branch already allows to download while installing other packages.
Cheers,
Sebastian