There is "The Whitespace Thing" :
http://people.csail.mit.edu/mikelin/ocaml+twt/

I have tried to reproduce something similar using Camlp4 twice, the first
time with a ad-hoc grammar reusing as much as the current grammar as
possible, the second with the specific goal of compatibility with the F#
"#light" grammar.
Both projets have reached a "good enough to test and use privately" state,
but I haven't actually made the effort to release them (knowing virutally
nobody would use them anyway, because it's camlp4). In particular, the
"pa_light" attempt never reached full #light compatibility.

If you care, I could clean the up a bit (more than two years of bitrot, then
probably don't even compile anymore) and upload them somewhere.