Draconis - the way to switch startup disks from OS 9 is a control panel called "Startup Disk" (Cunning that). Under normal circumstances, the control panels folder is usually listed under the Apple menu.
I suspect however that Destructospin is more interested in getting the B&W to recognise the external DVDROM and boot from it. Not 100% sure this is possible, but you might try booting the G3 with the option key held down. This will boot any modern Mac into whats called the startup manager, and will give you a choice of all the available boot volumes. Available volumes will be represented by a square icon each (either a HD or CD), and there are two arrows, one at each end of the volume 'list'. The curved arrow will rescan the bus, the straight one will boot from the highlighted volume. Just click to highlight the one you want. I am not certain the B&W is new enought to support this feature, but it should do.

The B&W was the first ever Mac with firewire on board, sadly it is also the only firewire-equipped Mac which doesn't support firewire target mode.

DVD-ROM drives are quite cheap second hand you know. Or perhaps you can find someone with a CD copy of Panther. Much simpler.