As requested, there is a manager API. I have linked you to the topic.
You will have to ask the developer whether he thinks he will implement this new HMRC requirement into the program at some point in the future.
But to be really fair to Manager, he cannot really answer the question if HMRC are still in the proposals stage. This quarterly reporting may never actually come into place as HMRC will come to realise the logistical hurdles for getting this to work. I personally believe that HMRC will spend millions on the project - then realise it doesn’t work and then abandon it.
As such until HMRC have actually made it clear that a: these are the rules that will actually be implemented and b: the rules are actually implemented in law, I cannot see any accounting program let alone Manager bothering to implement this into their software unless this Quarterly reporting already applies to some business sector and secondly applies in several countries as most software developers will not drastically change the program just for one country. Thats just my opinion however and I am not one of the developers so I cannot say.
I just simply do not believe that this HMRC requirement will come to pass because HMRC will not be able to design their own software to get this work properly.
But Manager supports API functionality so you can write the API if you need to yourself.