Is it possible to use an LDO as an Auxiliary source to a smps?
I'd suggest you use a 3.3V LDO with a series SCHOTTKY diode such as the SDM40E20LS or SDM40E20LA dual diodes. These diodes have very low Vf values of around 310mV @100mA.

Connecting two in parallel will get you about 1A capability at 410mV drop.



The LDO may have some minor supply current (about 10mA) if your SMPS drops by 0.1V during normal operation, but otherwise should tend toward zero. 

The two diodes in parallel won't share current exactly, but well enough to keep the load voltage well above your brownout level.