It should work fine as long as you use a very low loft batting. I can't imagine trying to fit a cotton batting in that size harp. I have a 5.5" harp and barely fit a twin under it. Maybe the Quilter's Dream Request would work, but that would be the only possibility for cotton. Hobb's Washable Wool would definitely fit, just make sure you have a large flat surface to support the weight of the quilt while you're quilting it.

I quilted my avatar quilt (Queen size) along with many others and had no problem with my Bernina 1530 that has a 7" harp. It is planning on visually what order sections should be quilted. I start from the center and work outwards on the side that is rolled up (right side)

I have done a large King on a small harp machine (brother cs6000 and I would guess around 5-6"). I did fmq and straightline quilting. It isn't the easiest process, but it can be done. I puddled the quilt and had support for the quilt behind the machine and to the left of the machine. This eliminates having to fight the weight of the quilt too. You don't need an expensive machine with a huge harp space in order to make a beautiful quilt.

Oh goodness, I thought my six inch throat space was small. King size but would only do one like that in sections in future.

I've done lots of large quilts with a small throat space, but it requires a great deal of surrounding support space, which has already been mentioned, as well as a lot of rolling and pulling through. My hands would cramp afterward from all the rolling and pulling. You do have to work from the center out as half the quilt rolled up is really pushing the limits of space. Good luck!