To be blunt, I feel quite strongly that this is a bad idea. You might find business in this service, but only in companies which don't know any better. And if you can sell a service like this to that type of company, everyone (you, your client, their candidates) would be far better off if instead you sold them consulting services which made them more competent. Give a man; teach a man... right?

To explain:

Remember, companies are just formalized groups of people. What's the single most impactful component on a company's success? The people which make up said company.

The best candidates know this rule intuitively. They've completely internalized it. When these candidates imagine an excellent place to work, they imagine it first in these terms. These candidates will read the use of a service like the one you propose as a company shirking responsibility in an incredibly key area.

Yes, yes... it's only a first round interview. You're just separating the wheat from the chaff, right?

Maybe the company knows that you/your service is/are incredibly competent at this task. However as a candidate I sure don't, and my first impression is going to be that the company relies on a third party to determine whether or not it's even willing to talk to a candidate? I'm sorry, but I'm outta here. I don't have time to market myself to two separate entities just to work for the final one, especially when I'm already being recruited by two other companies which are competent enough to validate their own candidates.