Custom grid lines with tiles and GIS data

I’d like to plot GIS data with a map background and meaningful axes (latitude, longitude).

There are plenty of examples using data converted to Web Mercator projection ( with tiles added in the background. These examples all turn off grid lines because meters from lat, lon = (0,0) is useless to an end user. GMapPlot will render with latitude and longitude, but isn’t truly open source and currently still has the pixel offset issue.

Is there a way to transform coordinates in the axes from Web Mercator projection to latitude and longitude (or better yet, any other arbitrary transform)?