Re: do I have enough clearance?
I just had a quick scan over
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. I am not seeing anywhere in there that says you must mount it in the vertical plane, only that it has to be horizontal and clearly visible.
As long as your plate is visible and legible, you should be fine. Horizontal means you can't mount it top to bottom (so it reads up or down rather than left to right), but you're not doing that. Your only issue is one of visibility, and as long as the angle does not make it unreadable, you should be fine.
If it were me, I would just run it at the normal angle and not lose sleep over it.
Whatever you do, don't endanger your ride.