I'm still having a problem finding where Roosevelt originally proposed it as voluntary.  I've read the original proposal, and it contains a compulsory contribrution as well as an optional additional contribution for an annuity for those that wished to do so.  The optional additional contribution idea was left out of the final bill.  I am not sure if this is what you are referring to, or if there is something I have missed in reviewing the proposal and fireside chats proceeding it.

The optional additional contribution proposed for an annuity is somewhat similar to the recent ideas floated regarding a set aside of a portion of contribution for self directed investment.

From my reading, the Trust Fund still exists in the form it always has, funded by employer and employee contributions that must, by law, be invested in various government securties.  The question I think you really are raising is whether this investment of the Trust Fund is risky given the deficit in the general fund.  That I agree with.  Although US government specific and general bond issues are historically about the safest bet around, it is getting a bit scary now.  I'm curious as to whether you support the "lockbox" idea floated a few years ago.