What would happen if the end of a 7.62mm blank crimpped round was sawn off?

There is a potential for unintentional ignition of the powder inside the cartridge. If you're dead set on doing this, you first need to uncrimp the end, and (safely) dump out the powder. Once this is done, you want to thoroughly clean out the cartridge and remove any residual powder. An ammonia-based solution (such as household window cleaner) would be good to use here.