I do not know if I have understood you well. But I do not see any problem to manage to be charmed with only a part of the arrow.Only it depends on the conjuration that you want to imbue for example:
if you want that the arrow head which have Sudden Light spell that Render useless a group of enemies. you do not have any problem. or a ball of fire, or a vacum, etc
Example of treasure companion: Jan the Alchemist might create small glass beads which have Sudden Light spells (5th level spell from the Magician Base list, Light Law) imbedded in them. Because
Jan cannot yet create a Daily V item, Jan decides to make five Single-use items instead. Jan further keys the items to release the Sudden Light spell when they are broken. Jan?s idea is to loosely sew the beads onto the front of his robes and then just pull them off and use them as needed.
Unfortunately for Jan, after offending the village smith at his local tavern, he is tossed head over heals into the walls of said tavern. The impact crushes the beads and releases all five Sudden Lights simultaneously. While this does bring the bar fight to a sudden stop, it also sends poor Jan deeply into unconsciousness.
other topic are the enchantament tipe arm. if you want that an arrow of yours hand crossbow once shooted change in a big Lance(TWO FORM WEAPON ). or Worse once strike in a mace ( what big hole , Though a problematic use). them the enchantament is over all de arm like rasyr saids.
(this tipe of echantment was used in one book of rolemaster to convert small knifes in two handed sword once Thrown
Paragraph of the book:
For the sake of economy, however, GMs may allow their
Alchemists to create partially enchanted items. An item
that is partially enchanted only has the relevant parts for
its purposes actually worked with the appropriate Make
and/or Work spells. Thus, only a sword?s blade, an axe?s
head, a ring?s gem, etc., would actually be enchanted. This
leads to a large cost savings, though some items will be
weaker for it (and GMs should be sure that this fact is
always a danger).