If you are certain that an embedded table of assigned countries won't work, here's another option ... Create a few multi-select fields and concatenate them with a formula field. If all 192 countries might be selected, you would need 10 or more multi-select fields, since you can only select 20 items from such a field. Suppose you organize them geographically: North America, South America, Eurozone, Western Europe, Southeast Asia, Pacific, etc. Then create a text formula field like this: ToText( [North America] ) & ToText( [South America] ) & ToText( [Eurozone] ) & ... etc. The result will be a semi-colon-separated list of contries in a long string. By the way, this would also eliminate the need for your join-table, if its only purpose is to establish the many-to-many relationship and not, say, to do other reporting.
Having said this, I still recommend looking very hard at using an embedded table of countries within your form. You can print the form with standard tools to get the equivalent of a printed report showing the relevant countries related to a particular record, for example, and you can email the record as well.
