Load capacitance estimation for compensation of switching power supplies
The LM20323 is a current mode control switcher, so you should only have to worry about the pole caused by the equivalent load resistance (\$R_L\$) and the total output capacitance (\$C_o\$).  There is a lot that is not known here and I am going to make some simple minded guesses about the situation.  


The board is a multilayer PCB, about 5"x8" with 60mil separation between ground plane and +3.3V plane.  \$Z_o\$ for the power planes would be less than 2 Ohms, and inductance across the span of the board would be less than ~3nH.  
Loop bandwidth of the +3.3V supply will be less than 10kHz.


With a board like that and with relatively low loop frequency, capacitor esr will dominate.  Yes the loop will see the total distributed capacitance that is connected directly across the +3.3V plane to ground.  You should put the compensation zero in the error amp to cover that pole formed by the total distributed \$C_o\$ and \$R_L\$.  

Some other things you should think about:  


It is kind of unusual to have more capacitance distributed rather than at the supply output (at least I think it is).  Say half of the capacitance is distributed.  That means that half of the 500kHz ripple current (like 300mA or so) is also distributed.  Is that going to meet the system EMI requirement?  It is pretty common to have a secondary filter in place to reduce the ripple.  A (adequately damped) secondary filter can keep the ripple currents confined in a small loop area to reduce EMI.    
If the PMIC being supplied by this 3.3V output is a switcher itself, have you looked at output impedance to input impedance relationship between the two?  Switching regulators have negative input impedance.  Unless the source supply has lower output impedance than the input impedance of the load switchers the system can oscillate.  It is best to have source output impedance about 1/2 the load input impedance.  This kind of oscillation can even happen between the input filter of the switcher and the switcher being filtered.