Assign default program to scanner button
Finally found the solution:
Control Panel\Hardware and Sound\Devices and Printers\Epsone Perfection V500
Right click => Scan Properties
Tab Events
'Prompt which program to run' is selected
Select 'Start with this program' => done.
The issue was -imho- not too localized as it applies to many devices with events. It is not a Epson problem to my opinion, but a general customization in Windows, which I failed to discover up until now.
Looks like a simple solution which could be googled in no time, but I have tried back and forth to find it before posting the Q. The fact that it could not be done with standard win7 settings just bugged me to the bone, so I guess I went the extra mile.
Thx everybody for thinking along.