I wouldn't limit a Repair Weapon skill to "weapons-based guilds." Weapon repairs are crucial to every guild (as combat leveling is crucial to members of every guild) and simply maintaining a weapon is not something that it takes dedicating one's life to learn. Anyone who can learn to use a weapon competently should be able to learn to upkeep it.

If some form of limitation need be placed on such a general repairing skill, perhaps a (non-tinker/fighter) player can only repair weapons he/she has skill in wielding. To simulate knowing how to repair weapons that he/she knows how to use. Tinkers and fighters could repair any weapon, regardless of having skill in using that weapon.