Should Congress decide to void one series of Treasury notes (the ones behind the Social Security Trust Funds), who believes that there would be no impact on the valuation of other treasuries held by other parties?
The SS trust-fund is only book-keeping entries. As such, there cannot be such as thing as a default. That's like your left-hand pocket owing money to your right-hand pocket.
The players in the bond market are not naive, they know that the same entity (the federal government) is both the lender and the lendee, and as such cannot default to itself.
Look at what's going to happen. When SS cash receipts aren't enough to pay out benefits, they'll go to the Treasury and say "We need $X." Since the Treasury doesn't have $X sitting around in cash, they'll have to go to the bond market and issue notes. Which is *exactly* the same as what would happen if the "trust fund" didn't exist.
Heck, there is no need to even formally "default". By legal definition, SS payments are set by law and the amount is determined by Congress, and that amount can be changed at any time by just changing the law. Just like the way they cut SS benefits in 1986 by subjecting them to income tax. And when they unilaterally increased the full retirement age.
Actually, when they go to tap the trust fund, the game will be up. It will be clearly obvious to everybody that the trust fund is just paper, when they have to issue T-bills to pay out SS benefits -- just like they do for every other Federal expenditure.