get rectangular bounding box of oblique mercator projection

Greetings,
Is there a simple way to get (or plot) the rectangular bounding box defined
by an oblique mercator projection? For example, in one figure I'm using

-JOa-115.9/33.5/135/9i  -R-116.2/33.7/-115.6/33.4r

This is a rectangular area, tilted at 135deg. relative to North. I want to
show the outline of that area as a line on another plot. Two of the corners
are defined by the -R command above, but I need the other two. I could
probably do the math by hand (intersections of great circles?), but is
there an easier way to do that in GMT?

Thanks, Eric

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