PIC battery backup

Hi ,
I am running a 16F886 @ 5v and I want to place a pair of 1.5V dry cells as a 3V battery backup with a low Vf schottky blocker. I don't have any pins free, but I was wondering if I place a 39K resistor inline with the primary supply (12V DC) to the TX RS232 pin and use that pin as a digital input to sample the voltage if that would be a reasonable way to monitor the main supply. The built in PIC clamping diodes could handle the 0.3mA overvoltage load.

I need the PIC to detect the loss of main supply and enter a current saving mode to just maintain a clock.

When RS232 ops are called for I can momentarily stop the voltage monitoring to execute comms.
Would that 0.3mA pull up affect RS232 comms?

in my RS232 dual driver i have a few transistors attached to the RS lines both side to run leds, so personaly i cant seem your 0.3mA making any difference to any but the very high rates, but i dont know for sure just that they run leds via transistors fine and mine ive had running pretty fast.
Also dosnt RS458 use a pullup or am i getting confused?

I think I would be looking towards using one of Maxim's or Linear Technology's excellent devices for this. Virtually no switching noise, very low dropout etc etc

If its the tx pin 300ua probably isnt going to affect it much.
You can use a pair of fets as a diode for the battery supply with very low vdrop.