Hi Deepak,
Only thing I could think of is to play around with the UoM and perhaps define a new custom unit for Hours which is equal to a 100 (or more) standard hours. The new UoM can be labeled hours so from user perspective they wouldn't know the difference. There are couple of master switches such as default assignment unit that you might have to change. I have not tried anything like this before, so you may have to do some testing to make sure it does not have any undesired impact anywhere else.
The other option which you probably would have though of already would be to do some custom development and let the system over allocate by default.
Just out of curiosity, what is the business reason for this. If you are intentionally over allocating resources, that means the resource management functionality is not being utilized the way it should be (i.e you are essentially saying that you have infinite capacity, and if you do have infinite capacity there is no need to manage resources to begin with). I believe the system behavior in this instance is correct although a master switch or config setting to control whether the system over allocates by default would have been nice.
Regards,
Lashan