This idea will take more planning and will be more costly because of the plane tickets, hotel and other travel expenses. If that's all in your budget then this can be a very thrilling and fun out of town proposal. Next, you will have to discuss this and get permission from her boss. If her boss agrees to the plan, he will ask her to fly out of town on business.
Her boss will also notify her that unfortunately she must go alone because "company policy" does not allow her to be on business with anyone unless they are your spouse. If this is untrue based on her past coworkers who have gone on business trips with their boyfriends, girlfriends, her boss can add that this policy has just recently been in place. You will be making all the arrangements with the hotel and notifying them of your plan.
You will arrive at the hotel substantially earlier than her to prepare and make sure that she does not see you in the lobby or nearby the hotel. Once you get the keys, you can prepare the room and have the option of making the room look romantic by having candles and roses throughout the room for when she arrives. She will be pleasantly surprised to find you waiting in her hotel room upon her arrival.
You can propose to her as she arrives or you can wait and greet her and slowly explain to her why you are in her hotel room. Regardless of how you will do it, this will be one proposal that she will remember and share with her friends and family for a long time!