I use the accessory bar below the handlebar and mount the bar bag on that, on a clickfix bracket. I also made myself a map holder to fit the same clickfix bracket so can use that instead of the bar bag if I want a lighter front end. If the accessory bar is a similar length to your stem (I think they come in two lengths), you probably won't have much trouble getting into the bar bag, but that might depend on how it opens.
The Computer is mounted in middle of handlebar. I use inboard bar ends which are just on the bend of the comfort bars. There's still plenty of space.
If your new thorn is a Raven, the main point will be getting the shifter and brake cables behind the bag and not snagging if the steering turns right round. I ended up using a loose cable tie in the end. But that comment might equally apply if you were mounting the bar bag on the handlebars. On the tandem a pleasant side effect of going Nokon with the cables is they are now flexible enough to allow the shifter to be rotated so the cables come out the bottom instead of either side of the brake lever, and don't get in the way at all.
I run a Schmidt dynamo hub and Solidlight on the tandem, but made an extra bracket to hold a little Cateye reflector / flashing LED on the fork crown bracket under the Solidlight. If you have enough headtube / spacers and use a bar bag which isn't too deep it can all work together.
