You would need to work in an environment where jQuery Mobile (or alternative - jq.mobi, Sencha Touch, etc.) is loaded, and used during rendering of the previews or WYSIWIG views. You might want to see what tools are available for developing regular websites using JQM, etc.
I see your viewpoint. But I don't think you'll find what you're looking for. It would have to be a more stable and cohesive environment (which Microsoft has).
Best to work on strategies that allow you to better separate concerns. So, prototype pages using a browser, provide some dummy data (what does Visual Studio do about that? Sorry - I haven't used Visual Studio in years...), then take the result and turn it into .erbs. You replace the dummy data blocks with <%= > tags.
You could perhaps automate some of this. If you keep logic out of the .erbs, it could be as simple as coming up with your own proprietary tags, and using a general-purpose macro processor. You could come up with a few proprietary tags. You (or your designer) writes HTML with these special tags into, say, a .erbpre suffix. The macro processor can generate either an .erb, a .html, or both. When generating an .erb, it would replace the proprietary tags with <%= %> tags. When generating an .html, it would replace them with dummy data either from files or from a test data generator.
You might want to see how Rails developers deal with this issue, since Rhodes has much in common with Rails.
I worked on backend admin stuff in Rails at a major console gaming studio. For admin stuff, programmers designed the pages and generally worked directly with the .erb file - they don't need to be so pretty. For the public websites (leaderboards, etc.), a designer did it using Adobe tools, using dummy data, then handed the HTML to a programmer, who turned the .html into .erb. No, the process doesn't provide instant feedback, but feedback in that situation was a couple of cubicles away.
Here's a page that I think describes the situation and offers a solution - albeit for an obscure framework written in Scala: