Let me try too.

Well, how I see the possible interface solution for the subj.
a) The Custom Columns settings dialog will have some additional feature to show, if the columns set is general or WFX-plugin-specific.
Possible solution (fake screenshot - 14 kb).
Description: the list-box contains all the WFX-plugins installed, plus the first item
General (or
<General> or something else) which means usual custom columns for usual files/folders. When you select an item from this list-box, the custom columns list changes to one that belongs to the plugin selected (there should be a special list for every FS-plugin).
b) When you select the
General item, the Edit Custom Columns dialog remains absolutely the same to its current appearance and behaviour. When you select one of the plugins, the dialog looks the same, but pressing the
[+] button will show the menu consisting from only 2 items: standard
tc item and the currently selected WFX-plugin's name. See the
fake screenshot No.2 (24 kb). The plugin is responsible for the list of the possible columns.
But it should be possible to create custom columns even if the plugin does not support its own custom columns! For example, I may wish to see for one plugin the file size (in addition to name+ext), and for some other - file date. Both size and date can be taken from the internal
tc "plugin", so there is no need in supporting any additional column by the FS-plugin itself in this case.
c) And the last: how to select the custom columns in TC itself. The
Shift+F1 menu remains the same, but when the FS-plugin is active, this menu shows only the column sets for this plugin only. See the
fake screenshot No.3 (8 kb) for example. Of course, when I select Configure Custom Columns from this menu, I get the same Custom Columns dialog (as on the first screenshot) with the current plugin selected.
If someone has anything to ask/add/correct, I'm ready to discuss.
Added:
2
ghisler(Author)
There are some questions remain about the default behaviours and about the behaviour when switching to other plugins/modes. I'll try to cover them too.
Well, the most convenient solution could be the following: TC remembers the last column set selected for this partucular plugin (independently for all FS-plugins) and restores it when user opens the plugin again. For the first time (when no mode was saved yet) the plugin behaves as it currently do: it uses the currently selected mode (Brief/Full/...). If the mode is unsupported by the plugin, it may switch to Full Mode.
When switching from one plugin to another (or to a usual non-plugin folder), the custom columns mode may change to Full either (because this mode is common for all FS-plugins, as well as Brief and some others), so it will work properly in any case. Of course, if the plugin is currently in some compatible mode (like Brief) the mode should not change.