We are a small business with big customers. We alleviate their source code concerns by making source available in a git repository that is always accessible to them. The contract basically says that if our business stops operating that they have a license to the source to continue doing whatever they need. They can in theory, but may not, access the code in disputes. They have no day-to-day rights to the source, and they ensure that their own developers don't have general access.

Customers are happy with this arrangement, especially if you offer to make use an escrow service provided that they cover the costs. This way, it is 'free' because lawyers are only involved during the initial contracts. After that, their are only two parties, no lawyer trying to get ongoing fees.