Ubuntu :: X11 Keybrd Layout / Difficulty Getting Diacritic

I do a lot of work on Ancient Near East Languages (Hebrew, Aramaic, Akkadian, Hittite, Ugaritic...), and I need a lot of diacritic marks. Following these instructions and using the character map, I've managed to get everything I wanted. There's just one fly left in my ointment. There is a standard for transcribing aspirated Hebrew consonants which involves putting a line below the character.I can make them using a combining character (U0331), but this is the rub: with all the other diacritics, I have to add them before the character, and with this set, I have to add it afterwards. This doesn't seem like a big deal, but it's a huge pain in the butt when your actually using it. I tried using combining characters with all my diacritics, but they don't work right with capital letters, so that was a no-go The combining characters also do screwy things with formating and spacing sometimes.

For all the dead-keys, there is a normal kind of formating that X11 uses. A grave("`") is "grave", and a dead grave is "dead_grave". A macron ("�") is "macron" and a dead macron is "dead_macron"... not rocket science. Anyway, I've tried all kinds of possible names for this character with the word "dead" in front of them, and no luck. I've tried a fair number of Unicode codes, and no luck with them either. I even tried "dead_U005F" (U005F the normal symbol).