Not directly through Opera - as you know from your post you got no answers on the Opera forums.
However, a work around is to play a file in Windows Media Player on loop (muted if you like).
Or, some software called Don't Sleep can do similar.
FYI, Internet Explorer does stop standby whilst downloading - I know not an answer but it may be another work around.
A more technical fix (I know it is written about Vista but my research suggests it is the same for W7) but possibly not ideal as this application wide (ie not just for Opera):
PCs may not enter sleep mode when they have a file open over the network. Graphics-intensive screen savers can prevent PCs from entering sleep mode as well, and should be disabled.
Otherwise Vista should not allow software applications to stop it from going to sleep. However, it is possible for a network administrator to change this default, allowing applications to "veto" sleep when they are busy. The default can be changed using Group Policy, but cannot be set through the Power Options applet in the Control Panel.
- Computer Configuration
- Administrative Templates
- Power Management
- Sleep Settings
- Turn on Applications to Prevent Sleep Transitions (Plugged In).