It is possible. I'm at work so I dont have my rainmenter files to adjust the code and give to you but you could try it yourself if you know how to do the code work.
rainmeter has an "imagerotate" meter, and since the text is done through image, you would just have to put the ImageRotate=90 in with the hour and minutes time measure. From there it might need some adjustment with the X and Y positioning, but it will certainly work.