I think it might be possible to resolve this issue without removing the feature: if only a foreground color is being used, mIRC will check for duplicate foreground/background colors and fix the foreground color if necessary. However if both a foreground and background color are used, no check is performed.
Does that make sense/sound reasonable?
Yes, that sounds like the perfect solution. I have no problems with it changing the color to make it visible if no background color is used. It's only when the background color is used that it matters. If we use a background color, then it doesn't matter what mIRC's background color is because we're setting our own background color, so it shouldn't try to fix anything.
The good thing with the fix when no background color is applied is that Invision can drop Visufix, which also fixes text to make it visible, though it takes it a step further and tries to keep the color as close to the original as possible while still being visible instead of just making it the default color.