Seems to me a programmer setting a default schedule in an application is far simpler than many of the other suggestions I've seen for solving this problem.
End users do not have any interest in saving ISP upstream bandwidth,
they also have no interest in learning so setting defaults in popular software, for example RFC1918 space zones in MS DNS server, can make all the difference in the world. This way, the bulk of filesharing would have the defaults set to minimize use during peak periods and still allow the freedom on a per user basis to change that. Most would not simply because they don't know about it. The effects of such a default could be considerable. Also if this default stepping back during peak times only affected upload speeds, the user would never notice, in fact if they did notice they would probably like that it allows them more bandwidth for browsing and sending email during the hours they are likely to use it. I fail to see a downside? Geo.