...sigh...
here's a workaround (spent 2+ hours polishing it). It's a workaround, because it actually implements what you might need (non-wordwrapped RichMemo with horizontal scroll) by using external component TScrollBox and TRichMemo itself. (The proper fix should do it within CarbonRichMemo itself)
The workaround is using OSX specific APIs to extract necessary layout information. Thus if you're to use it in your cross-platform project, you'll have to use $ifdefs.
The workaround also recommends to use r4623 of RichMemo. The revision implements OnSelectionChange event support.
You might also notice that RichMemo is yellow and ScrollBox is green in the workaround project. The colors are set via object inspector and could be reset to default colors, if needed.
P.S. speaking of monetary rewards. You could donate some dollars to the community. The link is on the left side of this forum.