We just uploaded another solution to this problem :

opam init --comp 4.01.0+bin-ocp

It is a binary distribution of OCaml 4.01.0 ("ocp" stands for OCamlPro, as it includes a small patch for relocation of the executables), working only on Intel Linux for now (32 and 64 bits), so you will only pay the download cost (about 65 MB), and not the compilation cost (you will still need to compile the next packages that you will install, as it is only the OCaml distribution). On most of our settings, downloading is much faster than compiling.

Once it is installed, it keeps the archive in a cache (in ~/.opam/.ocp-compiler-cache/), so that creating other aliases from this switch will only be the cost of decompressing the archive:

opam switch 4.01.0-for-something-else --alias 4.01.0+bin-ocp

Tell us if you meet any problem using it !

--Fabrice