i would go for it and just keep an eye on the structural integrity of the supporting limbs. i might try to keep the overall platform weight as low and as "centered" as possible, which makes me think of at least two ways it could go down the road:
1)depending on the soils and the weight distribution of what's left for the tree, and the weight of the platform, a water logged shallow root system may fail long before trunk and large limbs decay, and cause whole structure to topple.
2) if weight distribution is even and direct over root system (i.e. the tree is not a leaner) and if root system does not decay that rapidly under water, the portions of the tree outside the water will decay faster than those under water due to sunlight, intermittant moisture, bugs, woodpeckers, etc.
what a cool project!! definitely post updates!
