How to get water out of the frame? Now the (seatpost and QR) has been practically replaced by an (internally routed riser setpost) tightened with a torque wrench.

I disagree with the accepted answer here - sometimes above 1 liter of water accumulates. And stays there for a month.

But the old advice of "remove the seat, leave bike upside down overnight" is not practical anymore. Now what?
Barring a drain hole somewhere,  a drain hole allows air to move so equalizes pressure. A sealed system will create negative pressure with temperature changes and suck in water around moving parts. The bottom bracket or fork are the only other places a typical frame has holes that would allow water to escape.

Given these probably about as inconvenient to remove as the seat post, the only real option you have is a drain hole. 

You could always upgrade :) to an external routed dropper so its possible to remove the post, or install a long enough dropper cable that you can pull the seat post, and either cable tie it in a loop after it exits the frame (I know, you are rolling your eyes right now.) 

Question I have to wonder is where is the liter getting in.