I once got approach by someone putting together a fund for just this purpose. The rules were something like:

1. You buy into the fund with your shares.

2. The shares had to have had a valuation by a major VC in the past N months (N=4 IIRC).

3. You had to retain X% (they didn't want founders dumping on the fund.)

Finally decided it wasn't worth it. It is very difficult to have faith in other people's valuations of non-tradeable stock.