In order to load an older version, the VI needs to be recompiled and the run-time engine does not include the compiler. For that matter, the exe/dll does not include the source code required for a recompile.
In any case, I don't understand why you need to 'carry around several runtime libraries' in the first place. When you deploy a 8.5 application, for example, all that you need is the 8.5 run-time. The only exception that I know of is with something like MAX. Some of the functions are written in LabVIEW and those functions may require a different run-time but that is included in the MAX installer. Can you give an example where you need multiple run-times?
I also don't see each run-time being that large. The 8.6 run-time is a little less than 100Mb (on my XP machine) once it's installed. The older ones are much smaller and the actuall installers for the run-times are quite a bit smaller.
