Thanks for the info Thiago. My problem had a different cause related to packet contents, and a timeout. These comments are about the Connectivity_TestApp.c example in the MKW2x IEEE 802.15.4 examples.
Because I was testing manually and my mouse clicking was on the order of seconds, I had to first remove the 80ms timeout.
//TMR_StartSingleShotTimer (RangeTestTmr, 80, RangeTest_Timer_CallBack, NULL);
Next, the packets I was trying to sniff weren't MAC 802.15.4 compliant, so I had to enable promiscuous mode prior to RXEnable:
PhyPlmeSetPIBRequest(gPhyPibPromiscuousMode_c, (uint64_t)1, 0, 0);
(void)MLMERXEnableRequest(gAppRxPacket, 0);
Then, since packets are further filtered in PD_SMAC_SapHandler(), I disabled the SMACPacketCheck() function call:
case gPdDataInd_c:
//if(FALSE == SMACPacketCheck(pDataMsg)) //software packet filtering, to avoid parsing errors
Note that commenting out SMACPacketCheck() is bad if you are expecting 802.15.4 packets. In my situation I knew the packet would not contain a typical MAC header.