Made a revision to this houserule after re-reading Divine Source. Since you must be at least 9th tier to take the ability a third time (the Guardian, Hierophant, and Trickster paths also each have an ability that requires 9th tier to be taken a 2nd or 3rd time), I added a line to the rule the removes the lower tier restriction at 10th tier. I also changed the starting tier from 5th to 4th and the lower tier adjustment to 3 tiers instead of 4. Makes the houserule a little more powerful, but I don't think it's too overpowered and it serves the purpose better to open up more opportunities to take Mythic abilities and feats.
Mythic Flexibility: Once characters reach their 4th mythic tier, they can take mythic path abilities and mythic feats in place of normal feats gained through class levels. They must treat their mythic tier as 3 tiers lower when meeting any minimum mythic tier requirements to take abilities and feats in this way. Upon reaching their 10th mythic tier, this restriction is removed. Mythic path abilities and unique mythic feats (such as Dual Path) may only be taken in place of feats gained from normal advancement. Mythic enhancement feats can be taken in place of bonus feats as long as the original feat was one that the character could take as a bonus feat and as long the character meets all other prerequisites, even if members of the class don’t typically have to meet the prerequisites for their bonus feats. For example, a 10th level ranger with the two-weapon combat style and at least 4 mythic tiers can take Two-Weapon Fighting (Mythic) as a combat style feat as long as he already has the regular Two-Weapon Fighting feat.