Currently any cut/copy/paste uses Windows clipboard.
The alternative is to create inside ModPlug a different clipboard for each type of data - pattern block, whole pattern, sample block or instrument envelope. It can overwrite Windows clipboard if needed, but I think that's not needed, because it can be useful only to transfer data to another ModPlug instance, and since ModPlug may open multiple documents that's a rare case ( exception is the sample data which is the wave data that can be copied from/to standard Windows Sound Recorder tool - I've just learned this

).
So, final user interface I think of should look like this:
On the main toolbar there should be combobox with number of currently active buffer. That should be duplicated in "Edit" menu with submenu "Active copy buffer" and group of radiobuttons there. Also there may be four (I think that's enough :wink: ) little buttons on GUI of pattern/sample/instrument editor that will change color if we copy something or select another buffer. If user clicks second button then second buffer is selected and block pastes from there and so on.
For selection of the buffer there will be shortcut keys like Ctrl-Alt-1. Also there will be
togglable off 
feature to hit Ctrl-C/Ctrl-V several times to copy to/paste from different buffer - that won't change active buffer number, feature will have it's own buffer counter. Fancy-Dancy-Cut&Paste corner rotation (if fancydanciness enabled at ModPlug options at all) should be done
after user tried to paste from all available buffers ( look what is fancydanciness in ModPlug v 1.17.02.46, if Relabs will keep that version number and doesn't strip the actual feature at all 'cause it's hackishly implemented and noone voted for it on forum :? ).
Also when switching between Pattern/Sample/Instrument tab active buffer number should be updated to last one used in that tab.
And also there will be good to update copy routines to copy
all instrument data, not just envelope.
Wow. A lot of text. Have you read through all of that? :shock: