Just thinking about it, it seems that in order for the sealant (silicone?) to adhere to the inner wall of the tube, the sealant would have to be applied to a tube that has not been in contact with the damping fluid, especially if that fluid is petroleum or silicone based. You might try sealing one end before filling the tube, then carefully filling the tube so as not to get any damping fluid on the last half inch of the inner surface (using a syringe or a turkey baster), then seal up the other end.
I would think that automotive silicone break fluid would be one to try, as its viscosity is unaffected by the sorts of temperatures in a listening room. Also, it will not evaporate and is non-hygroscopic (does not absorb moisture), so its damping characteristics should remain constant over time.
Another possibility might be to fill the tube with this stuff :
The helical twist in the flat wire shown in the photo is a good way of introducing a non-directional nature to the interconnect. A flat ribbon would otherwise be difficult to rout in more than one plane, and it is a neat way of avoiding mono-directional damping of the wire. However, this solution comes at a certain expense; the electrons now have a longer path to travel before getting to their destination. The trade off in this case seems well judged, as the effect of fluid damping on a round section wire would likely be less (the damping fluid presents less resistance to a round section if we imagine such a wire "moving" within the fluid) and would likely end up just laying on the inner surface of the tube. If I were using round section wire, I'd probably string on some low density open-cell foam spacers every 6 inches to keep the wire "floating" withing the tube (open-cell foam would allow the fluid to circulate, facilitate filling, and not cause air bubbles).
Good luck, and I hope you will post your findings.
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