I created a bootable USB drive using Windows with Tiger on it using PearPC and TransMac, for PowerPC.

Here are the instructions to create a live USB of Tiger:

1. Copy the Mac OS X Tiger DVD to an ISO file.

2. Download PearPC (and a GUI, such as PearPC Control Panel) and the 6GB blank disk image (for 8GB drive) or 3GB image (for 4GB drive). Download the trial of TransMac, too.

Download PearPC and 3GB/6GB disk images here.

Download PearPC Control Panel here.

Download TransMac (15-day trial) here.

3. Rename the disk image. For example, "tigerusb.img"

4. Install Tiger from the ISO using minimal install (uncheck everything but what's required) to tigerusb.img. (Make sure you go into Disk Utility and format the image. Use the volume name "Tiger USB" for this tutorial)

5. When installation is finished and PearPC restarts, close PearPC when the intro video ends.

6. Plug in your flash drive and open TransMac.

7. Select your USB drive from the list and right click and choose "Format with Disk Image..."

8. Select "tigerusb.img" and let it copy to the drive.

9. The bootable drive has been created! Plug it into a PowerPC Mac, hold down Option while turning it on, and select "Tiger USB".

10. Configure it like you would as if OS X was on the hard drive.

11. Enjoy your Tiger live USB drive!

This tutorial is for Macs that don't officially support booting OS X off of USB. I use this method because directly installing from the DVD to the USB drive does not work.