You do not have to dedicate a continuous time block by using the RefMux user module. AGND can be routed to the analog buffer output by software.
Even when a continuous time block is used by PGA or comparator, AGND can be routed to the output through the testMux of that block. The only requirement is that the output of the continous time block should be disabled so that the testMux can be routed. Use the following lines of code to bring out AGND. In device editor, turn on the respective analog output buffer.
or REG[ACAxxCR2],14h ;set testmux of buffer to AGND