In these circumstances, I believe it makes good political sense to keep it simple and just guarantee that no worker with personal accounts will get less in total benefits than Social Security promises today, rather than try to defend a complicated, more limited guarantee.
The classic example of a non-recourse loan is a simple mortgage (if you sign a personal guarantee in addition to the mortgage, that changes the nature of the loan).
Instead of a simple choice between autonomy and instant independence, they should be given the option of voting for autonomy with the guarantee that another vote would be held in three (or five) years' time on the matter of independence.