Hi Seamus
I'm no expert on this but have considered it myself as an issue.
Firstly I have seen two distinct varieties - taught or loopy ropes.
For a taught rope system I would eye spliced one end on treaded it through to the other end and marked on the rope where the middle of the last eye splice should be. Then this is the bit I'm guessing on you deflate the tube section to get you some slack in the tube so you can work on the splice easily, put in the eye splice and pump up the tube - if you have got the rope length correct it will end up taught but not pulling on the tube. As an after thought some ropes contract after the get wet for the first time and other materials stretch - bit like the webbing cam bands on air tank straps.
For loopy lines they normally they have tubes threaded through them and most of the ones I've seen use a loose weave hollow braid construction. ( looks tightish when taught but opens up when you push it back on yourself) These use a type of braid splice - this really is into the reames of your local rigger - or pick his brains after a few pints
I think its a case of a simple tuck splice with the end threaded back the center of the braid this needed to be quite long possibly 10x the diamiter of the braid. To hold the loops at the right lengths I have seen astich with the whole rope passing through the wall of the rope in such a way as the lock so it doesn't pull though no matter which way you pull on the rope. I would guess that to finish off it would be a similar technique as for a taught line but I don't know how they hide this end in the tube also.
Possibly one of the tube manufacturers would know or as I suggested it might be worth a few pints to your local rigger for some advice.
Hope it helps Jelly
