Tak - Maybe I didn't understand what you were saying, but I couldn't find a "Measure Lines" category; applying the default beat line color to Major Rules didn't seem to get rid of the measure lines, if that's what you meant.
However if I changed the "Major Rules" color to white, although that doesn't totally get rid of the measure lines because the PRV still has the alternative light gray and white horizontal stripes for the black and white keys, the measure lines become pretty much invisible. So I think that would accomplish what the OP wants and maybe that's what you were describing?
On a related topic, I appreciate your giving me a heads-up about the color preference for measure lines because unlike the OP, I've always wanted to emphasize them more. So I made them magenta, and now I can really see those suckers! Thanks. I also found that changing the time ruler numbers color to yellow isn't as aesthetic, but they're a lot more readable.