Usually that's a no-no, or not recommended at least, can lead to some undesireable reaction. 

Has anyone tried placing the mass bias in ones ball track???

Yes

I tried it once with a Tomahawk Xplosion, ball was the bomb, but my pap at the time was 3*1.5, so there's that lol.

Hammer has a MB on track layout in their asymmetrical layout instructions.  They refer to it as a Label Layout "Smooth Arc".

Actually, this probably depends on intermediate diff. If it's low, probably doesn't matter. Now if it's .030, that may be a problem. I actually laid out another ball like this, and the intermediate diff was .008, and it was smooth and a big ball. Perfect first ball on the house shot. Moving in was suspect, but keeping it tight without jumping off the friction was key.

The no mercy was a z spin ball I believe.... been so long and so many balls ago.  Putting the mass near the track lengthens the hook zone and tends to make an asym act more like a sym.....smooths out the transition from hook to roll.

The MB/PSA position acts like a second pin sort of. Rotating about the pin is the lowest RG and rotating about the MB/PSA is the highest RG.   The MB/PSA distance to the PAP can change the amount of flare and how the ball reacts.  On a symmetrical ball, if you put the pin on the track you will have almost  zero flare.  If you put the MB on the track,  that will also make the core not want to flare and remain stable.  But depending on the Pin to PAP distance, that could make the ball try to flare.  So the two can sometimes fight against each other and give a wonky roll.  More times than not it just reduces the flare and smooths out the reaction because it is going to usually put the pin at 1 to 2" to the PAP creating low flare and the MB/PSA on the track creates low flare as well.