that said, copy-pasting 4 commands to enable the repository is just not that hard, so i'm not sure if a 'custom solution' is worth it...
if anything, probably implementing this spec:
would be a much more productive use of time, since it would be useable by everyone.
hmm, in fact, i just think i'll try working on that in my spare time.
EDIT: it seems that another spec, https://wiki.ubuntu.com/ThirdPartyApt is further along the lines of implementation already.