With the introduction of the herblore skillcape perk from this update, you can make unfinished pots with grimy herbs as long as you have herblore skillcape, max cape, monkey, etc.
However, Zahur isn't coded to create unfinished potions for you if your herbs are grimy. It would be great to extend the skillcape perk to work in tandem with Zahur, i.e. wearing your max cape will allow Zahur to make unfinished pots with grimy herbs.