Suggestions
1. Remove the concept of minimum guild level to equip an item.
2. Add the word "slotted" to the name of all magic items which include a guild augment slot.
3. Rename the word "guild" from slotted magic items to something else.
4. Create separate non-guild vendors for augment slot crystals. Those vendors sell crystals that may have higher price, less duration, weaker buffs, higher min-level, or otherwise be inferior to the ones available to a high-level guild. Such crystals might additionally appear in random treasure.
Motivation
As explored in this thread, excluding non-guild and low-guild players from a whole new mechanic of loot manipulation does not benefit the game. If it is felt that players need additional motivation to join high-level guilds, then that can be sufficiently accomplished by providing them with a superior grade of augment crystals.
If slotted items are opened to be usable to all players, then the designers will have additional freedom to do things such as add augment slots to certain named raid items, which otherwise would be too exclusionary to nonguild players.