I am happy to discuss this when the time comes. Some questions to consider:
1. Do you really mean 'Essay questions', or is some other level of abstraction better. (E.g. what about OU's pmatch question type?)
2. Is this really a question feature, or a quiz feature? Presumably plagiarism services do better on longer documents, so all responses to a quiz as one submission might be better than doing one question at a time.
3. When, exactly, do we plagiarism-check the work. Think, for example, about a quiz in adaptive mode.
I am sure all this is soluble, but there are some design decisions to make.