[Anaraxes]

A sai isn't even a missile weapon, so Spell Arrow seems a strange place to start. What's your actual goal here? To have a way to carry a spell around on your weapon -- presumably so that it triggers on whatever you hit? To easily change out the enchantment on the weapon?

You could just mount a Spell Stone socket in the hilt.

The easy way is to have your friendly neighborhood mage that's going to be making those Spell Stones for you cast the spell with Delay on the sai. No special gemstones needed. The advantage of a Spell Stone is that it doesn't count as a spell "on" for the caster. The advantage of Delay is that you don't have to Concentrate for a second to activate the effect.

A weapon that just kept on reapplying the same effect without the need for a mage to "reload" it each time is just a regular enchantment.