LDO with a bypass option?
If you use a proper LDO then yes the 3V3 will go straight through like you say.  I think that you could get an IC that has a P channel mosfet as the pass element.  Check the specs for On Resistance of the pass device and if it is low enough you will be able to do this.  Example: 300 milliohms Rds(on) means 30mV drop if your 3V3 rail draws 100mA. If you are drawing lots more current then using discretes will give much less on resistance than an IC.