I have always said that there are issues that do not have a direct answer of Yes or No but the scriptures made it clear in Romans8:14 that “For as many as are led by the Spirit of God, they are the sons of God.”
Let’s take it one by one. In my perspective, modeling a new design of cloth that covers nakedness is not a sin. Modeling only becomes a sin when you’re promoting immodesty and indecency. I don’t see anything wrong in promoting a designer or a cloth line with sane designs. 
Now to the issue of beauty pageant competitions, it is a ‘No No’ for me.
In most beauty pageant competitions that I’ve watched, they wear several clothes and in most cases, they (contestants) are to wear body revealing clothes like bikini, swim suits, etc. I see it as another way of making people watch suggestive pornography. You cannot bring pretty ladies to cat-walk with bikinis on in front of men and not turn them on carnally. On the basis of that reason alone, I personally think a Christian shouldn’t be involved in our modern beauty contest.
I also see it as another way of giving ladies undue attention and exposure. There is no way you will present yourself for such a contest without attracting randy men to yourself. Men are sensual and mostly carnal in thought especially when they see an array of pretty ladies on display. For me, let your beauty be a revelation to your husband alone and not something to show case to the public.