Thats def a good idea. Though add a step in between when you clean the headlights. After restoring the headlights with the drill pad and the plastic x, wash the headlights, clay them (its seems useless as the clay would be very clean till the end, but the minute dirt that will be pulled out will take the sealant process a long way) and then seal them with the given plastic sealant. Also keep in mind that the sealant does wear off like most paint sealants, so to maintain the work in pristine condition you will have to reapply the sealant at least yearly, if not every 6-8 months(just as a precaution to maintain them in as fantastic shape as they would be immediately after the restoration)
You could seal the headlights with a permanent coating like Opti Coat headlights sealant. Its expensive no doubt, one might be able to get a new headlight(maybe a little more) but 10cc goes a long way, you can coat a few pairs (2 atleast) of headlight and forget about the fact that they will turn yellow (atleast 5 years). The link is
Optimum Opti-Lens Permanent Headlight Coating, headlight coatings protectants
Hope this helps.