Firstly, since the Raspberry Pi doesn't have an RGB out, you will need an ACTIVE (with a chip to logically convert Digital to Analog) HDMI to VGA. I use one of these on my RPi and it works great- for older VGA monitors.. but yet again the price is going up...
I bought this HDMI to VGA from ebay
- I have Model B, I do not know if it will work on Model A because of power constraints.
- Also you have to read this. Apparently using and inline active converter can cause long term issues with out tweaking something on the board....
- UPDATE: In revision C boards and older the power constraint is no longer an issue
So you can get this one suggested by Mark Booth if you are concerned about the issues described in the wiki.
Secondly, you will need to convert from VGA to CGA. Something like this bad boy! (known as a GBS-8100)
You can buy it on ebay for not that cheap.. But maybe it can help you develop some other mod along the same line?
Some additional references that might help you with alternative home brew solutions.
This looks like a converter but not sure if it actually does anything in processing the signal to the correct format.
Again another way to take CGA into Scart.. but maybe some reverse can be applied?