Updated
Dress for the rehearsal dinner you are actually having, then add the detail that makes the outfit feel bridal to you. A restaurant cocktail dinner, a garden gathering and a formal dining room call for different clothes, even when all three appear on a wedding-weekend schedule.
Start with the room and the dress code
Write down the venue, start time, dress code and what you will do after dinner. Emily Post's attire guide includes short cocktail dresses and dressy separates for semi-formal events, while black tie can call for a floor-length gown or dressy cocktail dress. Use the wording you gave guests as your baseline.
A mini works well when you want the shoes to be part of the look and the dinner has a cocktail feel. Separates give you another styling decision at the waist and can be easier to rewear as individual pieces. A floor-length skirt brings more formality, but check stairs, outdoor paths and how much moving between spaces the evening includes. Our mini, midi and maxi guide goes deeper on hem length.
Choose how bridal you want it to feel
Ivory is one route, not a requirement. A silk surface, corseted neckline, scarf or deliberate hair accessory can make the outfit feel connected to the wedding without copying the ceremony gown. Put the full look on one page with the shoes and outer layer; the coat you need at the end of an outdoor dinner is part of the outfit too.
For a Room 22 example, the Maxi Silk Corset Set pairs a scoop-neck corset with a bias-cut floor-length skirt and matching silk scarf. It suits a more dressed-up evening when you still want separates. Try it with the outer layer you will take home and check that the hem clears the restaurant steps in your chosen shoes.