I guess you mean by "coaching" is travelling by coach as I understand in the UK you guys have to do that occasionly due to the closeness of airports. How ridiculous to have a crew travelling by coach dressed in a tie. I do understand the company requirements, yes we have to maintain the standard of the company etc when we are paxing etc but even on a plane it's totally ridiculous. This is 2003!! Show me any latest fashion with a tie. Business dress these days is still on the conservative side but has a more liberal and relaxed attitude. I would certainly turn up for an interview in a suit and tie but paxing, no way.
As far as the tech crew are concerned I would argue the fact that we are all crew under the same banner, we represent the same airline and there for should adhere to the same dress standards when paxing. Aren't you a team?? Do you have CRM??
Chat to the relevent managers and get this sorted. You will probably discover it is a rule that was written years ago and no one has bothered to update it. With the right tact and assurances that no one will abuse the relaxed rule you should be able to get away with less stringent dress standards. Smart casual can still look as good as business when paxing.