> connect each end of the pair to the TX and RX pins on two PICs,
> or would using say a MAX232 as well help ?

If it was me, I'd want *something* between the PIC and the wire,
even if it was just a couple of transistors, (a) for protection and
(b) for drive. 1/2 mile is quite a length, and will pick up noise like
RF, EMI, nearby lightning etc if it's in the open

Since this is a one-off, you don't strictly need to follow any "rules",
just make it work reliably, so any way that gets the job done is OK

http://www.lammertbies.nl/comm/info/RS-232_specs.html

Cable length is one of the most discussed items in RS232 world. The
standard has a clear answer, the maximum cable length is 50 feet, or
the cable length equal to a capacitance of 2500 pF. The latter rule
is often forgotten. This means that using a cable with low capacitance
allows you to span longer distances without going beyond the limitations
of the standard. If for example UTP CAT-5 cable is used with a
typical capacitance of 17 pF/ft, the maximum allowed cable length is
147 feet. 

The cable length mentioned in the standard allows maximum
communication speed to occur. If speed is reduced by a factor 2 or 4,
the maximum length increases dramatically. Texas Instruments has
done some practical experiments years ago at different baud rates to
test the maximum allowed cable lengths. Keep in mind, that the
RS232 standard was originally developed for 20 kbps. By halving
the maximum communication speed, the allowed cable length increases
a factor ten! 

RS232 cable length according to Texas Instruments

Baud rate Maximum cable length (ft) 

19200 50 
9600 500 
4800 1000 
2400 3000 

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