Right now Prime's toon is lv23... which means he'll only be able to do the first two world bosses on Coruscant and Taris. He'd never be able to do the ones on Tatooine which is lv30 and on Nar Shaddaa which is lv32 (remember, we're doing the Tatooine one before the Nar Shaddaa one).
The reason for this is because, as you said, it takes a hell of a lot of effort to get people to these points in the game if they haven't already been there... and the second point is that once they get there, the person will be too low a level to really do anything.
All of their attacks will miss and a single aoe will most likely take them out.
So yeah, when it comes down to it, there's going to have to be limits placed, not only for those practical concerns but also because I'd rather not spoil worlds for some folks who haven't made it to those worlds through the natural progression of their toons quest line...
...unless they REALLY want to go I guess. If you really want to try and take your toon as far as possible we can try, but it'll be extremely difficult with no speeder points and being completely out-levelled by everything around you to the point where one or two shots and you're down no matter what you do.