The engine I'm basing my ideas from is BlackVoxel (
www.blackvoxel.com)
They track their world in sectors of (16x64x16) (tall boxes where x,z is ground); think I'm going to rescale that to 32x32x32 blocks (still only 15 bits)... but your position in the world is basically first divided by the sector size to get the sector and then the modulus is where in the sector you are.
So to extend their world to support independant floating sections, would just extend a sector to include a transform (or maybe wrap an array of sectors so I can have parts larger than a single sector).... I Like the idea of what space engineers has done with large and small ship frames.
There is like 0 physics support in blackvoxel, other than typically a player is 1x2x1 voxels so it's easy to see if you're colliding with something else. When I changed the world scaling to be like 0.25 the size it was, the the player was effectively 4x8x4 and since they were only testing the corner points, it was possible to go through a beam of voxels in front of you... so I set that aside for future consideration.
in the case of 32x32x32 the worst case of shapes in a volume is every other cube and turns out to be like 63488 ( ( 15 + 16 ) * 16 ) * 32 ... ( every other voxel, with an offset every other voxel in the next row times 16 times across a layer and 32 layers)
the best - stupid case - would be 1024 (32x32 cubes on top exposed, not building shapes for any voxels that are surrounded by other voxles... that have at least 1 face exposed). Could then join these in groups so make a single 1 voxel thick 32x32 voxel wide/long shape for the top. (this is a rare case though only useful for initial conditions)... and only useful for the areas around the origin (in black voxel as you leave the starting area they have various terrain biomes; one for instance is a forest of VERY large trees... which are probably 4096 shapes around a trunc through a sector (4 * 1024 for each side)... less than that 'cause it's round but... ).
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So I dunno maybe it is better to just do 1 rigid body for each sector and enable/disable them as they are required (say only 9 active at a time... or get really tricky and use only 4 at a time....) though that only solves collisions around the player... for each mobile sector they'd have their own 4 sectors plus all other mobile volumes.