Version:           2.0.0_devel (using unspecified)
Compiler:          gcc 4.4.1 
OS:                Linux

This is probably a post-2.0.0 feature, at least it is not blocking but rather nice-to-have. 
The idea would be to have more than one default config for the ASCII datasource.  We would use filename patterns as key to look up the rest of the config parameters. The checkbox at the bottom of the ASCII config dialog to "Apply these settings as default" would set the current config as absolute default it the filename pattern is empty, otherwise as default for the current filename pattern only. And the filename pattern lineedit ("Always accept files matching") would turn into an editable combobox: selecting a pattern from the list would set all other parameters in one shot. And this would of course be used when reading a new file: the default settings for the first matching file pattern would be used.
That is probably less work and more robust than automatic detection of the format (which would be nice, of course) and could possibly make the lack of automatic detection acceptable.