Flooring Tile - tiling over edge of bathtub

I am building a shower enclosure around my bathtub and then tiling it. I need to tile up to the edge of the tub (as opposed to the new drop-in tubs which have sit on top of the tile). I am using Hardibacker (a new, to me anyway, type of backerboard). How do I go about tiling this rim/edge around the tub. Do I need to put plywood around the edge and then cover that with Hardibacker/backerboard and then tile onto that. Or can I just put Hardibacker/backerboard? Or can I just tile directly to the tub itself (it is an old porcelain covered iron insert). Because I am building an enclosure, I probably will need to get some sort of support from the edge,so I am thinking plywood andthen backerboard. If so, do I just cut small edge tiles to fit that small edge all around (it will be the width of the plywood and backerboard). It seems like a pretty small edge and that the tiles would be liable to fall off or chip off.

Anyway, any advice anyone can give me will be much appreciated. Thanks.

Pete

I cannot quite picture your project regarding the trim tile adjacent to the tub & why they are required, but that does not matter much.
Anyhoo, tiny tile pieces should be avoided near a fixture. They will probably not last long as a water barrier.
Consider using Corian if you need filler / bullnose strips.