I don't think on-disk chain size is likely to become a serious issue in the near term, as disk storage is one of our cheapest resources. It also impacts network bandwidth too, of course, but again, I don't see these as the most critical resources. Relatively speaking, I think cpu and memory usage are the big things to worry about now.
At the same time, I do like to optimize things where possible, and I have been considering a new custom_binary operation, as an alternative for apps that want to lower the RC requirements for their users. But this might already be implementable via the current extensions feature, even. I haven't had time to discuss it yet with the other devs here.
IIRC
custom_binaryoperations already exist, though not really used.