I think one way of attacking the problem would be using the existing nbtree by allowing it to store the five btrees. First read the README in the nbtree dir, and then poke at the metapage's only structure. You will see that it has a BlockNumber to the root page of the index. Try modifying that to make it have a BlockNumber to every index's root page. You will have to provide ways to access each root page and maybe other nonstandard things (such as telling the root split operation what root page are you going to split), but you will get recovery and concurrency (at least to a point) for free.
Why not write it using the GiST interface - that is after all the entire point of GiST...
Chris
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