Thats an interesting combo Mathew.
But I don't see any reference to guidescopes or an OAG in your list.
have you considered autoguiding?
Autoguiding, to a certain extent corrects drive errors and helps with getting longer exposures. Not sure how well it'll work with the EQ3, but if you wanted to save weight, you might have to look at an OAG rather than a piggy back scope. I'd say that'll bring you very close to the 3.8kg limit. cost wise you'll have to add in another $280 odd for the guide camera unless you want to use a modded webcam, and the cost of an OAG or a finder guider.
Without guiding, you may not be able to get exposures longer than a minute or two before you see star trails.
Since your payload will be pretty light, less than 5kg's, you might have other options like the Astrotrac or similar portable mounts that support autoguiding.
http://www.teleskop-express.de/shop/...ST-4-port.html
http://www.flickr.com/photos/markpayne1/5383859201/
Weight: 1 kg (2.2 lb)
Load capacity: 15 kg
good luck.