As Franko stated, technically that wouldn't be a problem. Given acceptable quality components you probably wouldn't be able to tell a difference between component (YCbCr) and RGB - the VGA upscaler would probably add some slight blur though.
Additionally, you should check your TV for input compatibility - many accept component or S-Video signals as well. The latter would be very cheap to generate be hooking up an old S-Video genlock to your Miggy and far better than anything composite. May be slightly bulky though with the small 600.
Also, you can try running the RGB signal directly into the VGA input (with a bog standard VGA adapter for AGA machines). Apart from timing these signals are very similar and some flat TVs do accept 15 kHz RGB.
Thanks for the replies! Very helpful.
Thing is.. It's not really a TV, it's a big computer LCD TFT masquerading as a TV.. so it's only got VGA and DVI inputs. I hadn't really anticipated attempting to hook up a C64 and A600 to it when I made the purchase!
It does seem like quite a faff.. converting all sorts of different signals, but hopefully it should work! I'll wait and see how it goes.
In terms of a genlock, that sounds interesting. The VGA box has s video input as well, so I could hook it up that way, but I'm not entirely sure what a genlock is or where to get one?
There's one here:
http://cgi.ebay.co.uk/AMITEK-FUSION-VIDEO-GENLOCK-AMIGA-COMPUTERS-/250794245930?pt=UK_VintageComputing_RL&hash=item3a64807f2abut it's a bit vague about its input/outputs.
