i have one of the manfrotto umbrella mounts. i use it more as a way to articulate the stereo bar for different configurations then for an umbrella, but you really never know when it will rain, right? i think it will work for what you are trying to do, and have some measurements for you. First of all, the adjustment of this will go just beyond 90 degrees in both directions. Without a protractor, I would venture to say that it will travel from the straight in line position that is shown in your picture to approximately 100-110 degrees. As far as the ability to accept 5/8 spigots, the answer is yes, as long as they are long enough.
Okay, some measurements for you. the openings at either end are 11/16 inch, with a depth of 1 3/4 inch. The tee handled lead screw will clamp down fully leaving approximately 1/16 inch of unclamped space. the four tipped knob lead screw when fully inserted leaves about 3/16 left over. Keep in mind, there is also a knurled knob that clamps the umbrella hole, (which is 3/8 inch diameter) a little past half way. you will have to find a sweet spot where knobs are not interfering with each other, which should only come into play with the tee handle and the umbrella knurled knob, which are on the same side of the elbow joint.
I hope this makes some sense.
Now, for how I use this with clamps:
Home depot carries a C-clamp that has a third lead screw that runs perpendicular to the C. with very little effort, I was able to remove this lead screw, and low and behold, it is a standard 3/8 16TPI, like the standard microphone mounting hardware. this of course opens all kinds of possibilities, as I replaced the tee handled lead screw here, with a length of 3/8 16TPI all-thread. I also use some other hardware to increase the diameter a little, which fits into the manfrotto umbrella mount, and viola! I have a clamp capable of variable opening sizes (the C-clamps are sold in various openings), and with various lengths of all-thread and the manfrotto elbow, an unlimited clamping option. of course, this is just one clamping option of many out there.
I hope this helps a little! i will try to upload some photos to help out