I've always found form customisation/permissions to feel a bit of a mine field in MODX. I know it's incredibly powerful, really useful and generally brilliant but I just find it really difficult to use.
I'm sure a while back I saw a post/issue/blog/something talking about a permissions/customisation wizard which would help simplify this process. I can't find it again, but just wondering if it's in the works or I actually imagined it.
I think that it would also be amazingly useful to be able to "trace" permissions on an object, sort of a customisation debug mode for users. Like "inspect element" in Chrome's web dev tools, but showing which permissions are in effect on an object in the manager and which ones have been overruled. This would really help tracking down in short order where things are going wrong!
You didn't imagine it, but it never happened (though there is a little mini-wizard for Resource Groups in the core now). I was considering it, but it would have been a lot of work and soon obsolete because (I think) MODX3 will have a new permission system.
Your "trace" idea is excellent and it would be great to have a System Setting that appended messages with permission debugging information to "Access Denied" or (maybe better) popped up a JS alert whenever a permission check failed. Are you listening Mark H.?