You can't.
A share is associated with a work item - and it can't pretend to be associated with a different one.
That work item has a difficulty that the pool assigns.
The work item is of course also unique per 'worker'.
Ah, I see. That's what I was wondering before, if there was something in the work items that was unique per connection/difficulty. Thanks.
Well it's effectively unique, but by reference rather than being part of the work.
Yeah that is actually a 'bug' in stratum that I argued with Slush about in the stratum thread (and others mentioned but didn't care too much about it)
Slush's answer was it was too hard for him to code it ...
... crappy programmers everywhere you look ...
A workaround was put into effect rather than actually fixing it properly.
The fix was of course to make the difficulty part of the actual work, the problem was the pool sends out the difficulty as a separate message to the work and it doesn't specify which work it is associated with either, it's simply a "difficulty change" message.
The workaround is in the stratum spec about how a difficulty change is associated with what work.
... but no you can't game it.