Well, I tend to drift towards reallity wich means I have to restrain myself not to over-rule everything
so, as a GM, will state that:
1) If the throwing player is static or moving
towards
the target, then no malus added.
2) If the throwing player is moving and
not strictly towards
the target then add a -10/5ft at least.
All of us has thrown rocks (lots of them
), and broken branches as spears too, and some have even done some athletics and thrown javelin .... and we usually take 2-3 steps/mini-jumps to gain some momentum while focusing on the target. And all those steps are always towards the target.
So that's my 2 cents
Knowing that HARP is just a (terrific) rule compendium focusing in playability mor than a reality-like game I will use point 1) and 2) as my house rules, as well as 3) -20/5ft for firing while moving.