(Yes. The government is already investing social security money by buying bonds.)
I don't think that's how this works. This is my understanding: The money comes in generating an IOU. Some of it pays for current social security benefits to retired people. The excess is spent on current governmental expenditures (this could be anything). I don't believe any T-Bills or bonds are issued against the use of this money.
Actually, I was told by a fellow who worked for years FOR Social Security (and has written a book about it).....that they DO put the money into financial issues, like bonds.....