Juniper roots thicken very slowly so root over rock will take many years to look good even if you can find enough good roots.
For a tree to sit well on a rock you need plenty of lateral roots growing at around the same level but no main down root because that gets in the way of getting the tree to sit well on the rock.
You can try the ror idea if you like but I doubt it will turn out well. I would not even bother.
You can chop off up to half the roots on healthy juniper with no problem so I don't think it would miss the above ground roots here but hard to tell without looking close at what is under the soil. Sometimes they have been chopped severely before and there's few roots except for the ones you see above ground. Always check before chopping any roots.
The other factor to check is reverse taper. Trunks quickly swell where larger roots grow but it is not always evident until you chop the roots and expose that part of the trunk. look real close to see if there's any sign of reverse taper below the high roots. If you see slight reverse taper it is likely to look far worse when the roots are gone. In that case I would go the other way- plant deeper for a year or 2 to get new roots growing at the same level of the high roots then chop all the lower roots off leaving the tree with the high roots and new roots only.
Hope that all makes some sense.
