Nor is the howdah weapons, I think, "Mundane" weapons. I see mundane weapons as spears, hand weapons, halberds things of that nature, the howdah weapons I would call, at the very least "special" weapons. I think by Mundane weapons they mean weapons that can be added to characters or units, i. e. spears, halberds, great weapons, etc.
Logically, yes, I see no reason why he couldn't smear the venom on the barbs for the giant blowpipes, or the giant crossbow shafts, since he is up there, but...if he did that, from a logical standpoint (and yes I know, logic rarely plays in to GW rules), then it would seem anyone shooting it would gain the benefits. Therefore, I think that is why you cannot say it works.
I do tend to ramble, even when I know what my point is!!! Must be the former teacher in me.
So anyway, I think the operative word is "mundane" and what needs to be defined is...what is a mundane weapon. I know it is "non-magical" but does it include larger weapons that just the normal run of the mill "hand weapons.
That's my 2 cents.
John