A quote from page 7 of the preview, with WotC's indulgence...
"Assasins focus their shadow energy through a ki focus, which is not an object they weild but a reservoir of magical power within themselves."
"You can imbue your ki focus with magic as if it were a physical implement"
So it is a well spring of power within you that you can enchant.
However I do see your point about improvised weapons. So an assassin could use his ki focus enchantment on any weapon he was profiecient with that he happened across. But not improvised ones.
"Assasins focus their shadow energy through a ki focus, which is not an object they weild but a reservoir of magical power within themselves."
"You can imbue your ki focus with magic as if it were a physical implement"
So it is a well spring of power within you that you can enchant.
However I do see your point about improvised weapons. So an assassin could use his ki focus enchantment on any weapon he was profiecient with that he happened across. But not improvised ones.