I don't know if it's possible either, I'm under the impressionit isn't. :c
I think thedude found Barney on some planet that already existed in the LU so that's how he found him at least.
What's been confusing me is that in Concerning the Construction of a Certain Rad Washing Machine, at the beginning it says that he's already been to the SW dimension for blueprints, but the Undrive isn't built until later in the story. :S
I was considering the possibility that thedude stumbled upon a portal/gate/thingamabob during his journeys across the Universe (dimension). If we don't rule out the possibility of more than just one in a dimension it is perfectly possible that thedude found one in his travels, because he visited a lot of places. Or perhaps, unbeknownst to him when he was in Crux he was mysteriously (though he didn't realise it) transported to Militiregnum where he climbed some mountains found the very same gate Jonna did, went through it, returned and then was re-transported back to Crux, not even realising he had ever been elsewhere (besides the SW dimension). This is possible (though a bit far-fetched), because of the special connection I'm planning to show the two planets have. Because I have not played all my cards, yet, just think of the two Kingstons and their relation as well as the fact the same language is spoken in both planets.