Disclaimer: Never booted from tape myself :-)

> If I write a miniroot to a tape (cartridge type used on a Sparc 330) can I
> boot the mini root directly from the tape? 

Probably yes

> If so what commands are required
> to instruct the Sparc to boot from the tape (the machine is bare and
> currently has no operating system on the hard drive)?

b st(0,U,F)

Where U is the unit number and F is the file number on the tape.
Some PROMS use a strange mapping:

SCSI ID of TAPE		Value of U
	4			0	Suns default SCSI ID for first tape
	5			1

Then I have booted from CDROM (SCSI unit #6) with the command
b sd(0,30,1) on sun4 and with sd(0,6,1) on sun4c PROMS. Strange.

Harald.