I found a small tidbit in the October 4, 1949 [Lansing] State Journal:

The city council Monday night agreed to pay for the expense of boring about 350 holes in downtown sidewalks for use by merchants in displaying flags on standards in front of their stores. Merchants previously had asked that the holes be put in the sidewalks back from the curb so they could display Michigan State college banners and American flags. 

Most of the sidewalks downtown appear to be of recent vintage, bearing stamps from the 2000s or later. Still, I wonder if I can find any sidewalks downtown still showing evidence of these flagpole holes. I will have to check next time I’m downtown.