can I specify a time to mount a drive?

There's nothing you can do when the machine is asleep. Period.

The only thing you can do is schedule (via Energy Saver preferences) your machine to wake at the appropriate time (or shortly before), at which point your request is just a matter of normal task scheduling. For this you should look at launchd since it is the preferred way of scheduling tasks to run at specific times.

Either way, AppleScript can't handle the task scheduling for you - it can perform the tasks you want, but it can't handle the scheduling component.

I have to wonder, why do you care about mounting the disk if the machine is asleep - it's not like anything else is going to happen while the disk is mounted, either.

Well that still requires the machine to be awake, which can be handled via Energy Saver preferences.