While this does not technically answer the question (not using default buffer) it does solve the symptom of the problem so I thought I would still share. I get around this issue with a solution to a different problem.
I have mapped "Copy, paste" (yank, put) from the system clipboard to "Ctrl-Shift-C, Ctrl-Shift-V" (Ctrl-C, Ctrl-V if caps lock is on). This can be used in place of y
with the same effect.
If I use the system buffer for copy it does not get overwritten when I paste.
I added this to my .vimrc
vnoremap <C-V> "*p
vnoremap <C-C> "*y
As a bonus it will give you easy access to the system clipboard.