Recomendation for Riots API.
We players use Titles as indicators of skill. Tiers and Division. Your MM uses MMR.
Positional MM as a concept would contribute to making a Title a wider range of 5 individual MMRS, this does not read very well for players when we try to judge skill off a title.
May I recommend you add an indicator to your API that shows a Tentative Title? https://imgur.com/a/HekEh5m