I thought this was intrinsically linked to the alpha maps for the interface bitmaps? In other words (I am no way the expert here, try asking jesse, who knows their stuff about HUDs) you may have to change all the bitmaps to have alphas that are white, as opposed to midtones - that's what (I think) makes them semi-transparent.

There might be another workaround that doesn't require re-doing the entire interface, though. Get a second opinion first!

One thing i did when i made my Halo 3 Oracle HUD was double up the background bitmaps of shields and health bars. If you're on about other parts of HUD (grenade, weapons and blah), then i recommend you do the same. Ammo meters can have they're own background in the static properties, i think.

Haven't touched HUD in a while :|