yeah i see no issue with sloping the post stirrups - the only show stopper is the fact that you slope on two planes & this stirrup is fixed on one axis; you would need to check out the post itself & this would reduce the overall size of your post to the neat dimensions of the remainder of the post in the area of the cut (the now weakest point).
you could rotate the post until the angle is right for both planes of slope however this would mean that your bearers will not bolt neatly onto the side of the post.
consider yourself up on the amount of work required to dig through the concrete (since you have decided not to do it :U) - now you need to decide between setting threaded rod or grinding conc. as loki suggested - choosing the one requiring the least amount of work.
i think both would be fine structurally. grinding conc takes a decent grinder & discs and drilling holes takes a decent masonry pilot & bit of sufficient size & length. both these bits of gear can be hired; both methods will work well & more to the point both will be ok'd retrospectively by an engineer if approval is an issue.
r's brynk