These are just some thoughts I had. Please do not take this an a disagreement with the concerns expressed in your OP, or the need for modesty in weddings (and everywhere else), because I agree with you wholeheartedly there. But I am not sure about the manner you seem to be suggesting in going about answering Jessica's question.
IThis thought process permeates our culture. "How far can we go?" is the most obvious one. But we see this in every aspect of life, where professing Christians push the envelope, attempting to walk the tightrope that represents the line just shy of avoiding sin. They are focused on themselves. We should make it our habit to avoid even approaching the line as much as possible.
I don’t want to say anything on the specific issue of strapless dresses, but I am not sure that this principle can necessarily be applied in a blanket fashion to the issue of dressing (whether at a wedding or elsewhere).
The bible does not present the issue of dressing as being simply how far can we stay away from the line of sin, because the bible itself says that there is virtue in dressing well both in general (Proverbs 31:22) and in the specific context of a wedding (Is 61:10, Jer 2:32). The bible itself would say that God is glorified in both the attractiveness and the modesty of the bride’s dressing.
So I do not think it is really possible to answer the question of what to wear on a wedding without discussing the merits (or lack thereof) of a particular item of clothing under question. To ask “What will glorify God?” does not, of itself suggest an answer as the God glorifying response is not always the most “conservative” one (please remember again I am not making these comments in the context of any specific item of clothing).
That said, if concerns have been expressed about an item of clothing, I think a woman who wants to glorify God will want to avoid offence and will be willing to change her clothing choices at a wedding if she is asked to. That, I think, is the how these questions should ultimately be resolved in a church.