It's a Small, Small World
Thorn Longswords are bonded weapons that are forge-grown in the Royal Gardens of Caer Caelan. When initially bestowed, they are Masterwork weapons, identical to a normal longsword in every respect.
At any time thereafter, the Thorn to whom they belong can spend xp and time as if he were a spellcaster with the Craft Magic Arms and Armor feat to further enchant the weapon through intense meditation in any thicket or briar patch. No monetary investment is necessary.
The Thorn’s character level is used to emulate caster level as far as spells necessary. Note, this will not allow a Thorn to cast spells he does not have actual access to into a Spell Storing weapon, even though he can create the Spell Storing weapon.