CURRENT STATE AND PROBLEM:
You cannot directly worship Mythrien Sarath if you are a cleric or have taken the Gift of Devotion. You can worship Mythrien Sarath as a non-cleric non-GoD character, but if you take a cleric level it will remove Mythrien Sarath as your patron deity.
This means it is literally impossible to consecrate an altar to Mythrien Sarath.
PROPOSED FUTURE STATE:
You can directly worship Mythrien Sarath as a cleric only if you have the Gift of Devotion. This would make it possible for altars to Mythrien Sarath to be created.
JUSTIFICATION:
Per "Magic of the Seldarine," the Dragon Magazine article from which Mythrien Sarath originates, he is stated to have no typical clerics but does have specialty priests - Seekers - and crusaders (Dragon Magazine #251, page 30). He is not the only god who has no typical clerics:
Melira Talaren also has only specialty priests and particularly devout bards as lay clergy (Dragon Magazine #236, page 15), but we do permit clerics of Melira Taralen.
Fenmarel only has specialty priests who are canonically referred to as shamans (Demihuman Deities, page 112), but we do permit clerics of Fenmarel.