Yes alloy frames can be checkered. If anodized it is best to strip it first since it will be acid dipped prior to re anodizing which may dull the points. 20 line per inch on alloy would make it pretty susceptible to damage/ bending due to the height of the point. Some I have seen were 25 lpi. If the frame was being Ceracoated the bead blasting should suffice.
Most alloy frames are done in 30 lpi for that reason and due to the thickness and strengths of the frontstrap in the valley of the checkering. Aluminum also tends to gum up hand files terribly. I was taught to never use stones or files on brass and aluminum. I have not done any alloy frames and think most people will only machine cut them but I may be wrong.
Hopefully this is my last as I plan to get a mill soon and will try my hand at machine cutting and some other patterns. I have a new in the box stainless Custom II waiting as a project gun. I wanted to start with a base model.
As far as hand checkering I would say I put in 8-12 hours on one depending on the borders, height and if the trigger guard radius is reduced and cut higher. I am sure there are those that do it much faster.