Army Stud - contrary to the popular opinion in this thread I agree with your logic. In theory, you should be able to dilute your saliva considerably at first, let him increase your injections (quantity and frequency) and then dilute it less and less with each assay until you have reached a level of T injections that you are happy with. Then keep the dilutions constant. That way, according to the assays he will think that he has succeeded in reaching a normal physiological level of T according to your saliva, but the results are actually being skewed consistently lower than your actual T levels.
I would do the same thing. The advantage is that the T would be pharmaceutical grade and free. Worth the hoops IMO.
One problem with this scenario however is that it could be difficult for you play this game. I mean - trying to find the right level of dilution of your saliva to achieve a happy level of injections could be difficult unless you are seeing your results and there is a direct linear correlation between the dilution and the assay result.
Second problem is back to your original question - what to use to dilute the saliva. I'm with JDSoFla here. Get a shit load of saliva off a clean buddy and use it.
I am far from an expert and I am not familiar with the saliva assay so just take this as my ignorant opinion (though it does seem sound)!