The way you stated this is a bit strange.

Let's review that a "backdoor Roth" is the combination of a nondeductible IRA contribution immediately converted to Roth. So, there never is any time for earnings, and there is no Tax owed on that type of conversion. And it only works like this because there also is no pre-tax contribution or untaxed earnings in a Trad IRA (or account of similar type) that would trigger pro rata taxes on the conversion amount.

Is that not what you are considering?