Quoting directly from the Boosey & Hawkes style guide, page 98:
Tempos are aligned with the left of the time signature or, if there is no signature, to the first note or rest in the bar.
As I say, I’ve never seen a publisher whose house style is truly that they prefer to align tempos with the first beat of the bar even in the presence of a time signature.
And given that other scoring software doesn’t align tempos according to this (as far as I know) universally followed convention already, I don’t see why Dorico not supporting it and requiring the editor to manually adjust the tempo if that is needed should be a deal-breaker. By which I mean to say that editors are used to having to move tempo marks in scores produced in other scoring packages to make them align with time signatures, which Dorico always does without fail, so it shouldn’t be a deal-breaker if somebody perversely wants the opposite. If it’s idiosyncratic notation, it’s not our responsibility to provide every possible option, since adding options is not a zero-sum game. Every option takes time to implement, time to test, adds localisation cost, adds ongoing maintenance burden, and so on. So I will need some more persuading before we add this option, I’m afraid!