In case somebody is dealing with similar problem this solution should work (thanks Filip for helping):
Imagine the game is rendered in fixed virtual resolution. However you want to support drawing on various different resolutions (for example when developing for Android).
If aspect ratios match we just stretch everything to fit monitor. However when aspect ratios don't match we don't want our game to be stretched because the end result looks bad. This effect can be seen on TVs - if the broadcasting is in 4:3 and you have wide-screen TV you want to have black bars on sides. This is exactly what I wanted to achieve.
To get this effect working in MonoGame you can create scaling matrix - which is passed to spritebatch.Begin method; and setup new viewport. This should draw everything correctly including UI created in EmptyKeys, however, hitboxes are going to be off.
To fix hitboxes you have to change this class (if you use MonoGame): http://github.com/EmptyKeys/UI_Engi...seState.cs
Rewrite properties NormalizedX and NormalizedY so it uses your viewport and then build your own version of the library.
Hope this might help someone