As an addition to your change see the 'list view controls' in the appstore. This might help you!
I have done something like this in the past with an enumeration. Instead of showing the caption of the enum in the datagrid you show the enum icon. In that case you need to build an icon for each progress percentage that is possible(in my case just 25,50,75 and 100).
A other option would be to change your datagrid into a listview or template grid. Then you can use the progress bar widget.
That is a bit of a dilemma.. build 100 different icons for each percentage.. or use a list view. The list view is working with the progress bar. But unfortunately the search options are limited. Also, there is no way to sort it like in the grid. I made a table so it looks a bit like a grid, but it has not the same functions. Another point is that there are some buttons which perform some actions (delete, go to, edit etc) which are good in a data grid, but not really working in a list. So, that leaves only one option and try to make it work in a grid. I think i will limit my progress up to 10 parts (0% - 10% etc.). And then not showing the number within this icon, but just a image of a (part of a) progress bar.
Thanks for now. Hopefully Mendix can make this more easier in the future.
It does look nice though.. even if it is in list view