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I don't think that those items are guild-locked in this game, I don't remember seeing any indications of that.
I've asked someone to add a related question to this month's Q&A, hopefully it will get a look or some clarification.
I'm assuming you mean the usual 'high skill solo player who has the highest contribution to a fight' scenario, and asking if that player has the option to get the good drops too or if it will automatically go to a guild.
In Ashes, there may be some additional concerns here. Basically, 'spoiling'. It might not be possible for a player who chose the profession "Master Farmer" for their normal play to do the 'solo boss down' and actually have the skill/item required to get their preferred/correct loot off the boss.
You might need a Guild with the correct Artisan type to gather the item, and if solo, then you can't do it. As for whether this applies to things like flying mount eggs, it was asked, but we might not get a clear answer since the question has a lot of prerequisite parts...