Madstad make excellent products but you still need to match to a screen or take their complete solution which is HUGE and expensive. it is easy to slope the screen more towards you using spacers behind the (850) screen mount.
It is possible to buy the bracket alone, and fit it to the BMW screen. There's no reason to think it would not work with other after-market screens - such as the WRS, provided the screen is not too contoured around the mirror/handguards.
The bracket is available here - https://www.bikehps.com/acatalog/Madstad_Screens_and_Brackets_for_BMW_F850GS.html