If I understand you correctly, you want to set Smultron to be the default app for .txt files. This is actually supported by the Mac OS, and can be done for any file type/application, similar to the file associations in Windows.
Just select a text file (any file with the .txt extension) in the Finder, go the "File" menu, and select "Get Info". In the Get Info window, you'll see a section called "Open with:" From that area, you can select the application that you want to be the default for this type (in this case "Smultron"). Click on the "Change All..." button, and you can make it the default application for all text files.