Will roasted corn work as soup croutons?
Yes, toasted corn works great as a soup topper, especially for "American" soups, like chili, tortilla soup, or cream-of-tomato.

The difficulty is that, as you surmised, the corn needs to be first cooked and then dried and toasted in the oven.  Also, small corn kernels don't work very well since they become almost microscopic when toasted and dried.  Here's how I've done it:


Bought dried "giant corn", or hominy of some variety from a Mexican
market.
Soaked corn overnight in cold water, then boiled it for 20 minutes.
Drained corn and then roasted it on a cookie sheet at 200F for an
hour or so.


This is a lot of effort, and for that reason I never did it again even though it turned out well.  My easier effort suggestions would be:


Start with canned hominy (have not tried this to see if it toasts well)
Find a natural foods store which will sell you unsalted "corn nuts" in bulk, then toast them briefly in the oven.