I can't imagine any reason to not at least ask for their opinion as far as wearing the kilt. Hopefully they say yes, but if they say no you have already solved your problem as far as whether or not to worry about a jacket. If they say yes my thoughts are that since you are already planning to do the conversion finish that and then decide if you think it came out well enough for a wedding. That is unless of course you are pressed for time. Then you may just need to rent the jacket so that you don't finish the conversion, decide it's not nice enough, and not be able to get a rental jacket.