Is it possible to have a Section 83(b) Election when the only risk of forfeiture of the restricted stock is for unvested shares and there are no repurchase rights for vested shares (ie all vested stock is secured)?
Yes. That's the normal case.
In general, whether a share is "vested" means that the company CAN'T repurchase it under the equity plan.
Now, many companies have provisions where the company can repurchase vested shares if the employee leaves, but that's usually at the fair market value of the shares. That doesn't impact the 83(b) election.