I suggested DMAA because it is the only banned substance I can think of that is contained in legal supplements.
DMAA is only banned in-competition so I thought the "pushing the boundaries" comment might be referring to how close to game day supplements were being used. DMAA is allowed out of competition but obviously the more you take and the closer to game day you take it the higher the risk you will be caught with presence of DMAA on game day.
If a banned substance had been picked up during testing, we would already know about it, because ASADA typically returns testing results within about 3 months.
There are only a few substances which are actually banned by concentration, for example pseudoephedrine and T:E ratio. Pretty much everything else is banned at any detected concentration. Pro-hormones might lead to an abnormal T:E ratio but pro-hormones that actually work are likely to be illegal in Australia anyway. DMAA on the other hand was legal until very recently. There is no way Essendon FC was dispensing anything that wasn't available over the counter.