I can think of a couple of ways to achieve this but unfortunatly they would not be standard out of the box filters I'm afraid.
Depending on your level of expertise with SQL you could try:
1. Creating a custom SQL view that pulls back the total census data for each country along with the original data.
This would allow you to categorise the countries as either 'Above 20k' or 'below 20k'
2. Create parameter containing all the countries with over 20k census data and an option to select all. Use this parameter to filter the dataset either returning All or an individual country.
Feel free to pop me a PM or email if you would like to run trough this in more detail.