Yes, the AV Input is a miniplug just like a headphone jack.
What you need is an RCA to Miniplug adapter, like this for instance:
http://www.amazon.com/3-5mm-Mini-Plug-Computer-Stereo/dp/B000FEHO0U/ref=sr_1_2?ie=UTF8&qid=1288230347&sr=8-2Plus you need (2) Male to Male adapters for the RCA connection unless your RCA2Mini has female RCA inputs.
Hook one color to the video port and the other color to one of the audio ports on a RCA - out setup. You will get mono sound, you could probably use and RCA Y connector to the audio side and maybe even get stereo
Makes the Express/GT a full audio/video monitor for your Duo/TGCD/CoreGrafx/etc...
And yes, you can use the Turbo Vision with a GT, and a GT Tuner with a Turbo Express!
Here is the SKEEEEERY setup (DracXSpecialHuey):
Note for the pic, I used the GT Tuner with the Turbo Express to prove that it works.
Of course, this setup works for any NTSC source with RCA out.