Can't you simply adjust the H Pos / H Size pots on your monitor?
Alternatively, edit mame.ini and set
monitor custom and
crt_range0 to the preset specified here, for your monitor:
http://forum.arcadecontrols.com/index.php/topic,116023.0.htmlNow, edit the front porch part of that preset to the value you want.
Note that changes made in ArcadeOSD aren't applied in GroovyMAME, as switchres creates the best modeline it can on the fly, using the monitor specs given in mame.ini. You can disable switchres in mame.ini to get the changes made in ArcadeOSD, but then there's a good chance the video mode won't match the original game's (specifically, the horizontal/vertical refresh rate will be out).
[edit] Having re-read your post, I think you have already tried this? In that case, I'm stumped.