Not tested, but you could use a projected CRS to make your grid and then deproject the resulting shapefile to lat/lon geographic. This will distort the hexagons though.
In my part of the world (about 1-deg W, 51-deg N) 50 metres on the ground is roughly equivalent to 0.0004530 of a degree of latitude. 50 metres on the ground is also roughly equivalent to 0.0007170 of a degree of longitude. Using these values with the mmqgis plug-in it was possible to make a form of WGS84 lat/lon hexagon grid.
The attached picture shows such a grid, on a projected map image that is badly-distorted as a consequence of being reprojected on the fly to EPSG:4326.
But I would not trust such a grid as far as I could throw it. For what it's worth, my suggestion would be that if possible you change your project CRS to a projected one and use the mmqgis plug-in to make your hexagon grid using units of distance.