Erik,

Yes, I see what you mean Vdd-Vee cannot be >15V. Typical RS-232 voltages would require +12/-12 V on Vdd and Vee respectively, which of course is > 15 swing. this limitation also applies to inputs. If the 4052 were moved back and connected to the logic levels of the UART,before you convert to RS-232 or RS-485, then the 4052 approach might have a chance of working.

Jacob