Here's something I came across on a board
a while back that met rs232 voltage specs.
positive voltage supply for rs232 came from
12V unregulated supply.
For rs232 negative supply it used the
crystal oscillator out line,
buffered it with transistor,
followed by a dc blocking cap,
then ran it thru a diode/cap voltage
doubler to generate negative voltage.
( a 2 mhz switching powersupply! )
I assume this would also work at
11.059mhz with a 8051/8052.
( it was on a 68hc11...)

It's also possible generate the rs232
positive voltage supply from +5V
along with the negative voltage if you
only have GND and +5v for supply voltages.