TTL to +/- 12V conversion - what's a better way?
Use an RS-232 transceiver IC.  You don't say what the 12V tolerance is, but you might be able to get rid of your DC-DC and linear regulators if the +/- voltage from a charge-pump IC (max232) is good enough.

Otherwise, just connect the supply directly to a non-charge-pump transciever.  Use MC1488/SN55188/SN75188, which has a 300 ohm output impedance, so your added series resistance would be 700 ohms.