If the OSB is spongy or swollen, it should go. Trusses are generally engineered for an L/480 rating, so while you should verify that with the manufacturer, you should be okay for ceramic, but not stone tile. Any subfloor works best if it covers at least two joist/truss bay - that keeps the ends from trying to pull out of the end as it flexes inbetween.
If you are going to remove the subflooring to replace, you may be able to extend the drain to allow use of a standard 12" rough-in toilet. This will give you a greater selection at a lower cost. You didn't mention the spacing of the trusses...if they are at 19.2 or 24", then you'll want two layers of subflooring to support your tile. A second layer is always useful (except for the height!) to stiffen up the subflooring for tile.
I'd want to use some blocking so the edges of the subflooring are supported. You should probably contact the truss manufacturer to discover the proper technique to not compromise them.
Check out www.johnbridge.com for help in tiling. Especially in smaller rooms where the difference in cost is minimal, an antifracture membrane makes a lot of sense, especially where it can get wet - they allow you to essentially waterproof the subfloor. I've used and like Ditra from www.schluter.com. Neat stuff, and easier to carry, cut, and install than any cbu. Hardieboard is a fine material and you can successfully use that as well, but look into Ditra...I think you'll like it.