Given the comments in the
store web browser thread, perhaps it is worth adding a second checkbox to the chooser dialog where the user can select which browser to use. The checkbox should be a second column so that it can be sorted & selected. The store plugin would provide the default for this box. I see 4 values: force system browser (amazon and company), force calibre browser (any?), use system browser, use calibre browser. The first two would show as greyed unchangeable (un)checked boxes. The last two would show as a normal box, checked as appropriate.
A harder-to-use alternative, but one that might pay some dividends in the future, would be to have a way to easily get to the plugin's configuration dialog from the chooser. A tool button next to the checkbox in the 'Use' column might work. If this alternative is used, then the value of the choice should be shown to the user on the chooser dialog.
Not metadata related, but you might also want to have set all/none on the column header context menu. Nicer would be a pair of tool buttons in a virtual row stuck at the bottom/top of the table, but I haven't figured out how to do that with Qt.