Apparently, this update from blackwingcat (well-known for his stellar efforts at keeping Windows 2000 relevant) makes .NET 4.0 programs able to use TLS 1.2 under XP. I don't have time to try it, but for those who do, here you are. This is an unofficial patch that modifies important system files, so make backups beforehand and don't blame him or me if something goes wrong.
Original source (in Japanese):
http://blog.livedoor.jp/blackwingcat...s/1986401.html
I only slightly rewrote the installer so that it works when extracted from this archive, does not check the system language, and applies all Schannel registry values. Everything else is his work. You'll need to reboot manually after NGEN finishes.