I have found documentation that implies that the ALTPLL can be configured to generate spread spectrum clocking. The Megawizard has a section under "Bandwidth/SS" that would be where this would be set up, but the function is "dimmed" and can't be selected, regardless of the device I select.
So is there a part that allows this?
The SS feature of ALTPLLs are supported only in Stratix IV and higher devices. You can refer to the PLL (ALTPLL) user guide's SS section (pages 19,20) here for more details :