I built a tdcs device in the past. Looking at tacs, it looks quite simple to put together a device to output 40khz @ 1ma constant current for such a device. I would imagine you would use either the Zeo bedside unit with serial cable and write some Python Code, or the zeo mobile with the open Android API (+Android code) to send a signal (via Bluetooth 4.0) to a microprocessor (ie. Arduino + BLE) to flash some LED's and trigger the tacs device for 30 seconds or more when a REM state is entered.
I might give this a try, seems quite simple.. I already have the zeo bedside unit, so if I can source a cheap 3.3v Arduino BLE and wait a month for my FTDI serial cable to arrive (from china) I'll write up some python code and write up some schematics for a tacs device with flashing leds.