Im sorry but I can't think of anything better than FMS-1021 (Flexible Polyurtehane Foam) to do this job. It's relatively light, and very tough.
No composites, no resins. I wouldn't even consider coating non-like materials like Al-Li or Ti with composites. The composite coat would fail upon rapid heating followed by rapid cooling.
The coefficient of linear expansion is just too different between these materials. FPS-1021 will accomodate fro large temp deltas.

Don't know the circumstances or the actual condition of the STS Center tank, but this type of material HAS a specific lifespan. Wonder when was the last time that those tanks were re-coated, if ever.