Please note I have NOT tried this on my webplayer yet, but I have been doing a lot of work recently with bootable USB flash devices.
First, be sure your flash device hardware is bootable. Most devices that are "USB Mass Storage" compatible are bootable. Another way to tell is if they are compatible with Windows 2000 and XP without needing a driver. Devices that require a special driver under XP or Win2K are usually not bootable.
By far the easiest way to make your device bootable is to use a Windows 98 PC. Install any necessary drivers for Win98 and then plug the device in. It should show up as a drive, we'll call it drive X:. Now, open a DOS window and do:
format /s x:
If you already have stuff on the card and don't want to reformat, do:
If you are using linux, you can also install lilo to the device. This is a bit more complicated and may vary from machine to machine, but if you are linux-savy it's not rocket science.