The curve of a given shield might make applying tinting a little problematic. That Sunblocker product looks interesting. Positioning is key though especially if you ride at night. I used black electrician's tape to make an arc across my clear shield. But I put it a little too low and had to ride with my head really cranked up to see downrange. I fixed that at the next rest stop.
But a helmet with the Steve Canyon visor is really worth the money. I can lower the visor (not shield) a skosh and it makes an excellent shade. Of course drop it down all the way and it's really great.
I wear Rx sunglasses so the visor plus sunglasses is perfect on the open road. A little too dark on shaded curves though. Another nice plus is riding with the face shield fully open to get some cool air but keeping the visor down to protect the eyes from debris and drying wind.