I'm really sleep deprived, but I think this might be a tough problem. While I could be insane, I think IDE drives implement "APM" support (from the not-so-old Wintel spec), which basically means one command to set the mode, and then the drive will buffer/call a wait on new incoming commands and automatically spin up...
...meanwhile, SCSI has had the ability since roughly day one, in the form of the 'Stop Unit' and 'Start Unit' commands - but they require the *explicit* issue of a Start Unit to wake the drive back up. Whether this is hard to do in AmigaOS, or it's simply that nobody's bothered (maybe someone's aware of a util?), I'm not qualified to say.
Spinning drives up/down can be a mixed blessing. Constant rotation can be 'healthier' than spinning up/spinning down - most wear and tear gets its chance to occur then - on the other hand, you do run the risk of spinning all lubrication off the platters, and/or, say, losing the windings in your spindle motor to thermal stresses when you finally *do* shut the drive down/try to spin it back up.
