I am fairly certain that creating a zenpack to provide this is going to be a fair bit of work; I can't help you with that. I was just hoping to help point you in the right direction. Here is another link I found from the RRDtool mailing list that may also be helpful to you:
http://oss.oetiker.ch/rrdtool/forum.en.html#nabble-td7083249
If you aren't wanting to roll up your sleeves and get dirty in order to get this functionality, your best bet is probably to spring for the Enterprise product. Image may be NSFW.
Clik here to view.
If you wanted to take a shortcut, you could probably just attack this from the command line using rrdtool against your device interface rrd files. Depending on your needs, that may be all you need, and the folks in the RRD forums may be able to help more with the details on accomplishing that feat. Again, it's going to require some basic scripting skills... but might be easier than creating full Zenoss integration?
Best of luck to you!
