I made a poll on the CM forums a few months ago about what resolutions people use. It got a bit screwed as I forgot a few important ones. The most used one is 1680x1050 according to it, followed by 1920x1200, 1920x1080 and 1280x1024
Easiest would be if you'd try to shrunk your custom mainbar into lotro's original mainbar size so you wouldn't have to touch the code for it.
If you do however want it to cower the whole bottom area for different resolutions it might get a bit tricky as you need to keep seperate skindefs up to date and users have to change some file names etc.
Tip: Use high Y value to move the bar to bottom.. it doesn't matter how high value you use as Lotro's UI won't let the objects overflow from the screen so you don't have to worry about different height values for different resolutions (just use something like Y="3000" or so). Same goes with stuff on the right side. If you wanna align it to left obviously X="0" then.. might save you some trouble.
Modifying the mainbar code has some disadvantages: Users need to relog to get it work properly. Swapping between windowed and fullscreen will dislocate the bar and to get it right again relog is needed.