Decoupling Capacitors on each VDD pin on a tiny 36/49 ball WLCSP/µBGA chip really necessary?
You do not need to place the decoupling capacitors as close as possible (or even directly) to the power pins. Instead, be sure to use a VDD and VSS plane. Connect the microcontroller power pins as direct as possible to these planes using vias. Then, place the decoupling capacitors to a place nearby where it suits you best. Connect these decoupling capacitors directly to the planes using vias, again. 

Why does this work? At the frequencies the controller is working, the power supply impedance should be low. Capacitors are low impedance in the range of approx. 100 kHz to 20 MHz. (Btw, at about some tens of MHz, the plane capacitance is slowly taking over.) But this only works if the caps are connected via a low inductance path. The idea behind placing the decoupling capacitors next to the power pins is to reduce the inductance between them and the power pins. Now, the plane has almost no inductance. Capacitor placement has almost no impact if you are using planes. We have successfully been using well defined "capacitor isles", instead of capacitor scattering, on relatively large PCBs. 

Another tip: Try to place the VDD and VSS vias of the capacitor close to each other, as this reduces the inductance from capacitor to plane.