Strange but I doubt he does this. . . and if he does it might be something that is kept under wraps.
I've wondered why more highly ranked QB's don't do this but I did have the thought that it's entirely possible they did, they just didn't make it public, they told the teams directly and the teams selected someone else.
When a player signs a contract, he influences the market. Doesn't really matter why McKinnon got $7.5m/year, just that he did. That's how agents negotiate. When you compare his usage and production to Mack, and add in the fact that his contract was two years ago, it's kind of an easy argument for Mack's agent to make.
I also think Drake at $7.5m/year is unrealistic, but I guess we'll see.
I don't think that any of this is likely to happen but to me what you are talking about is similar to either baseball's minor league or to the loan system in European soccer leagues. In the end the team doesn't get any sort of compensation for injured players, that's just a risk you take. The idea behind them is generally to get younger players with potential some playing time to develop them. But if we are theoretically sending Chad Kelly to the XFL to get some playing time, we are most likely not planning on him being a major starter that year.
But quite frankly I just don't think it will happen.