Technically what you need is an electronically variable AC power supply. DMX dimmers generally
use Triac based phase angle control. This is sufficient for controlling lights. However for your purpose you need a truly variable AC power supply.
- Variac + Servo Motor Based
- IGBT based static controller
All you need to do is use a micro which takes command from the RS232 to set the reference voltage and your circuit should take care of that. Both ways it is highly possible. I am sorry I cannot share code or schematic of my already done work because it is a commercial project. However can help you through this forum to achieve your goal.