Actually, you are 100% protected from a loss of market value, assuming you are buying bonds at or below par value. The day you buy the bond, you know the par value, so you are purchasing a guaranteed cash flow in X years. The change in market value of the bond does not change this.
You are correct that the current price of a bond may potentially be above par value. But I imagine these bonds would be largely relegated to high-yield issues and were issued a long time ago. It's pretty easy to not buy those bonds.
The simplest way to execute this strategy is to buy newly issued govt bonds directly from the US Treasury, which guarantees you won't be buying at either a premium or a discount.
I'm personally not a fan of TIPS for tax reasons, but to each their own.