Inexpensive Modest wedding dresses are hard to find in Southern California. So what usually happens is that the bride finds a dress she likes at a good price and then starts looking for a Bolero Shrug. Something like this at White House/Black Market: (Except for an LDS Temple Wedding it would be made of Silk, Satin or Taffeta, to try and cover.)
I have seen it at many weddings, The problems become first that you have to size it just so, so that it doesn't hide the gown. At that point you're hoping that the bride isn't going to move because then she'll be flashing her undergarments. Then there is the fact that a Strapless dress usually sinks down low in the back, so you will have to make the cut of the back of the bolero sink down low as well. Not a charming look, It looks like the bolero is mimicking the shape of the train on the dress, or was attempting to have a tuxedo tail.
A while back I started experimenting with adding bodice tops to Strapless dresses, I've done a few now. This is what I've learned. You need to draft the pattern pieces with the dress on the bride. The bride feel weird at first but then get used to you cutting fabric along her skin and pining everything in place while she's still wearing it. I forgot to take pictures or the process so here it is on the dress form. You also need to incorporate a key element of the dress into the top portion that you are adding, so that it doesn't look awkward. In this one I added rouching to the sleeves. Also make sure to attach the top to the dress, or you'll have the same problems as the bolero. Just topstitch the dress to the top you created. The Stitching line is only noticeable when you are looking for it, it doesn't show up in pictures and most people don't even realize it's there. It's worth the piece of mind to the bride.