I think that a comfort bike would be a LWB RANS or alike. Low pedals, precise steering, and, with a fairing, a pretty good scootin' machine for a cross country trek. If you average 50 miles a day, you'd be there in notime.
Personally, I couldn't tour 2-wheel'd. Although 50 miles a day would be just about right for me (maybe a bit more), hills would slow me way down and balancing a load on 2 wheels at a few MPH would have me all over the road. I'd take my Actionbent trike on a long tour with panniers. It might be a little more work but I would sure enjoy it a lot more. I'd probably also want a small fairing on it also. Even at 2 MPH, a trike can ride as straight as an arrow. Only hinderance I can see is those highways with the rumble strips which are harder to dodge with the wider track.
As for visibility? It all depends on how you dress up the machine. I have a flag and a nice bright triangle on the back of my trunk. People give me tons of room. Some even put a flag horizontal out the left side of the trike for greater visibility. Point is, you just have to be seen... It's not how tall you are.