It might be riskier (because you'd have to identify yourself with government documents) to plant a backdoor in a similar way at a large, proprietary software ve

It might be riskier (because you'd have to identify yourself with government documents) to plant a backdoor in a similar way at a large, proprietary software vendor like Microsoft. But I don't know that it would be harder. And in the case of proprietary software there would not be nearly as much public scrutiny, and the scrutinizing public would have fewer resources for inspecting it.