So it's actually a screen aspect ratio problem than a resize problem? I think there should be some ways around that... I havn't looked in that that much yet, since I'm not that far in to the production yet, but yes that's an issue i have to deal with as well at some point. If you're making a game in 16:9 format and displaying on a 4:3 screen there will be black borders under and over, but I don't really see it as a huge problem since most smart phones today are in 16:9 format...
Iphone 4, and iphone 5 differs in ratio for example, but if you really want to optimize your game for phones with aspect ratios other that 16:9 i don't think it would be too much trouble, just have to be taken in to account when designing the game, especially the UI. But i wouldn't worry about it too much since most smartphones use a 16:9 ratio. If you want to please a few% that has other screen ratio then i think it can be done properly without too much hassle.
Scaling a 16:9 game to fill a 4:3 screen vertically without black borders might crop out stuff that are to the left or right, (Probably mostly UI Stuff) but that can probably be adjusted for with events.
UI elements can be placed using these system expressions instead... if you want something to stick to a certain point, like the lower right corner you could probably use set position to: WindowWidth - 30 windowHeight - 30, that would probably work for any screen ratio.