You need a "competent observer" (who doesn't have to be a pilot) to look out of the window for other aircraft whilst you're staring at the dials. That I think pretty much deals with the legalities.
But it might be as well to talk to the controller on the phone first to make sure they're happy with what you want to do - they might otherwise, for example, assume that you can do other things which you might not have practiced, such as entering the hold or flying the missed approach procedure (which might be what the controller tells you on the day, not necessarily what's on the plate).
Whether it's actually advisable for you to fly head in looking at the dials, even in VMC, without formal instrument flying instruction I'll leave for others to comment on.
How much use such practice would be - normally you are not just trying to follow the navaid dials, you're also trying to keep the aircraft the right way up - is another matter. It seems to me that without instruction there is a risk that you might get into wrong scanning habits which you'll have to unlearn when you learn instrument flying for real ... but you might be in that position already, of course, with the flight simming.