“What if you get hit by a bus?”

Offer a license option, based on a single domain rights usage (assuming its a web based product). Tell the client to analyze his risk assessment based on the cost of the license or the implications of you 'getting hit by a bus'. If the cost of the license is less than the implicated cost of impact on his business should you 'get hit by a bus', then inform your client that their best option is to purchase the license and secure the code accordingly.

I would not place un-paid for or unlicensed code in escrow. The client has a simple choice to protect their assets by paying a fee to obtain access to it any time. Do not be forced into a position where you are giving up potential rights to your product to any business that has not paid for those rights based on a premise of the unexpected. Be firm and polite, but explain your position clearly. If the client is serious about protecting their business assets they would have no problem paying the fee to protect their source (assuming its not enormous). If their motives are other than that, you will soon find this out by putting this option to them. Do not budge on the issue of copyright to your product.

This is a common approach by businesses looking to 'muscle out' smaller businesses with such approaches. In my profession i encounter such claims on a regular basis. Do not entertain such requests but at the same time allow the client a viable option to secure their assets at no additional cost to you. The risk is with the client and the cost should be born by the client.

If your client is willing to pay for it, you escrow your code with a neutral third party. There are many firms that will provide this escrow service. You write a contact that specifies exactly when and how the source code will be released from escrow.

You basically place your software package's source code with a neutral third party. This party will release the source code to your customer under certain specified conditions. (One example might be your company goes bankrupt.)

There are hundreds of companies that provide this service. Here are just a few.

Iron Mountain

Escrow Associates

Escrow Tech International

Typically you sign a contract that specifies under which conditions your software source code will be released to the other party. These services charge a fee. You typically have the big company, that is worried about what happens if you vanish, pay for the escrow fee. You also need a attorney to review this contract and you need to consider those legal fees as part of your bid to this company.

Finally, note that while your client may be quite happy with the escrowed software, programmers know that the source code is only one minor portion of a product. Getting something to work with just the source code is going to be difficult at best.

See question:

access to source code if the start up fails

You can offer code escrow. He can choose the service and pay for it. Or he can hand you a boatload of cash and buy/license the IP

EDIT - I would push for selling a source license. However, you retain ownership and the license he gets is limited for specific use(s). You can get more revenue that way and he reduces risk.