you just need to find the correct ArtAssetID's and make a transparent .TGA picture (using alpha layers, there are guides for that even at this site somewhere)
for example you could make a SkinDefinition.xml file that would only have these lines in it:
<Mapping ArtAssetID="LetterBoxBottom" FileName="invisible.tga" />
<Mapping ArtAssetID="player_vitals_backdrop" FileName="invisible.tga" />
<Mapping ArtAssetID="opponent_vitals_backdrop" FileName="invisible.tga" />
That would make mainbar and vitals invisible though there are several elements on them that you need to process as well so there would be nothing left (like menu/bag buttons on the mainbar). Best way to find these is to download latest Skin Art Pack
and browse the pictures to get the correct filenames for tags.
There are several "minimal" ui's on the site too if you haven't noticed. with a little tweak you might be able to update those for the current version of lotro.