Is it possible to use a wipperman connex master link on a shimano dura ace chain? I read a comment on a site elswhere that said this was possible, but is it safe? Or safe-ish?
It should be safe as long as the widths of the chain and the master link are the same. That is, if the chain is 10-speed, the master link must be 10-speed, and so on.
From the legal perspective, I do not think that either vendor (Shimano/Wipperman) will accept warranty claims in a case when something would break. Unless they specifically claim such compatibility, of course.
From the practical perspective, as long as the pieces are geometrically compatible (the aforementioned link width and the pin/rivet diameter), they should work fine together.
From my experience, I've used various SRAM/KMC/Wipperman master links on various non-matching Shimano/KMC chains without issues. Things did break occasionally, but so is the name of the game for me.