It'd be a waste of time

unless you know what specific technical areas they need help on, say, a particular database that needs cleanup/normalization, a particular system that can be improved on. I suppose you don't know that or the proposal would come pretty naturally.

A shot in the dark has very little chance. You would think companies would love to have volunteers. Who woulnd't love free lunches. Not so, unless it's a very small business, they are overly concerned with potential liabilities, risks and privacies.