Well, it is POSSIBLE.. But, kind of like, "science fiction possible".. In order to do that, you'd need to know all the space requirements for all your files before you stored them. Which I'm guessing you won't know. So basically, no, you're not going to be doing this in the real world.
The best hack I can suggest is get a couple nice fat hard drives and use partitions to separate things.. That way, you may get in to having some stored files that are horribly fragmented, but you could keep your OS files relatively contiguous.. Of course, you can just run defrag to help deal with that.. Also, I'm not sure file system fragmentation causes any performance hits on SSDs, anyone know about that?
Alternatively, you could use a file system like ext3 that doesn't expose fragmentation issues to users.. Nor does it need to as it performs house-keeping on its own and thus you basically never have fragmentation issues.