We officially do not support .Net below 4.0 right now, but it should be possible. In fact, most of the missing pieces are already there for the portable library build (where we do not have System.Numerics and full TPL either).
I myself have no time (nor personal interest to be honest, as I don't see the point of using .Net 3.5 in 2013) to work on a special build for .Net 3.5, but if someone finds the time to make it work in a simple way I'd be more than happy to integrate it into mainline and start producing official .Net 3.5 builds in the future.
I myself have no time (nor personal interest to be honest, as I don't see the point of using .Net 3.5 in 2013) to work on a special build for .Net 3.5, but if someone finds the time to make it work in a simple way I'd be more than happy to integrate it into mainline and start producing official .Net 3.5 builds in the future.