Many of you suggested that I open a support ticket, so I gave that a try. Linode did respond quickly, but they only offered to "pass this feature request along to management for their reference."
Hoopycat noted that you can prepay for 2000GB of bandwidth for $200/month. But that seems mispriced because you can buy 10 Linodes 512's for $199.50, and those includes the 2000GB of poolable bandwidth along with a whole bunch of storage, RAM, and CPU time. And if I prepay a year in advance, that cost drops to $179.55. Buying the bandwidth by itself costs $200/mo even if I prepay.
So for now I guess I'll have to keep buying extra Linodes for their bandwidth allocation. I'm currently only using one of my four 4096's. Still, I hope Linode will consider including more bandwidth in their larger plans rather than capping them all at 2000GB/month. Given that a Linode 16GB costs twice as much as an 8GB, it should include more bandwidth. But it doesn't.
Finally, I'd like to thank everyone who responded to this thread. Even though my problem isn't solved, I'm glad to have at least gotten it off my chest

.