I agree with Sir82. We would tell someone who turned up for the meeting for field service dressed inappropriately to go and change or stay home that day. But, since the preaching work is a directive from J the elders didn't have the right to stop someone from participating in general due to overall dress/grooming. Even if someone was on reproof. It was even talked about that disfellowshipped persons preach and it would be up to J if he accepted their service or not; but of course the DF would not be allowed to join the witness group.
Of course, it's all crap and control mechanisms.