I've installed an SSD and for a number of big directories (e.g. Users\me\Downloads), I've been able to use the Sysinternals "junction" command to create a hard link on C: that references a copy of the directory on the hard drive that used to be C: and is now D:.
It seems that the winsxs\backup directory is an ideal candidate for this -- anything that thinks it's writing to or reading from c:\windows\winsxs\backup will really be working with d:\whatever\path\I\choose\backup, and it will of course be slower but files there are rarely touched (I'd think).
However, it doesn't seem possible to remove the directory in order to use to re-create it as a hard link to a directory on D:. Does anyone know a way around that?