That rod is a real pain in the butt, terrible. In fact I suggest you sell it to me at a fair price just to relieve you of the trouble of solving this problem.
Yes, the RPLX can be overloaded with good results. I have cast th10wt Rio clear tip aqualux floating / intermediate tip line on that rod as well as any of the higher grain (325-350 grain) sinking head lines like the Rio Deep Sea and they rocket out there. The floater w/ tip casts better with more line out but the loop forms well with short lines too. You just need to get enough grains out there before you can shoot line. With the sinking heads like the 350 grain deep sea that rod just turns into a rocket launcher.
I am building the VPS 4-pc 10wt Sage Travel rod which to me feels close to that 9wt RPLX travel rod. I used to have one but it was lost by Airborne, sent uninsured by a friend I had loaned it to on the Florida Keys. The VPS feels a little sweeter in the heart of the blank and I can't wait to finish mine. I've cast the factory built version and it's going to work out with the above Rio lines real nice.