If you want some suggestions on your proposal from a wise and analytical head who has also carried bags of cement on utility bikes with front racks, I suggest you sign up with the iBob list (http://search.bikelist.org/) and ask Jan Heine.

He should be able to give you some technical perspectives on the front fork and geometry that may influence how you go about finalising your design.

One of the issue I foresee is how you are going to maintain stability of the front end while you tip the material from the bucket. And for what I'm worth, I'd suggest that it be hinged, but then that will likely create issues either with (a) overhang and subsequent influence on handling or (b) clearance of the bottom of the bucket over the front wheel when tipping.

And while your weight might counterbalance the load when you are on the bike, what happens while you are off the bike and filling up with your load? StephenH's suggestion of a trike overcomes some of this issue.

In fact, while I appreciate your ingenuity, I think Stephen's suggestion of using a trailer might be a better solution. Or a wheelbarrow.