Sensing a USB device plugged in without having to supply 5 volt power all the time
You can't count on what a device will do when it doesn't receive valid USB power.  For example, it might not even turn on its pull up/down resistors to indicate the speed until it thinks it's plugged into a USB, which it might detect by looking at the bus power voltage.

If you can tolerate a little lag when a device is plugged in, wake up periodically and turn on the USB power for a little while.  One problem is that devices might themselves take a little time to come up once power is applied.  Some of my PIC programmers, for example, have deliberate waits of a few 100 ms after they run before turning on the USB.  This is to make sure everything is truly stable before talking to the host.  There is no guarantee how long you need to wait for a device to want to talk to the host after being powered by the USB.  A half second is probably good enough for most cases, but you can't know that for sure.

A more reliable strategy would be to make a converter that takes very little power when no current is drawn from its 5V output.  That shouldn't be too hard.  If done properly, the drain on a 5V output with nothing connected can be a few microamps or less.

USB is optimized for light weight devices at the expense of more burden on the host.  There is no free lunch.