For those of you who wanted to know, Riu has finally responded to my question about the day pass. If you are staying at one of the Riu you have to pay 45$ for a day pass if the wedding is at the palace. If you are staying at a completely different resort then you have to pay 95$.
Also, you can only eat dinner (a la carte or buffet) at your own resort. So if you are staying at the palace and you want to have dinner with friends staying at the Yucatan, you can't. After 6 pm everyone needs to return to their own resort.
They sent me a price list of some of the extras. Give me your email address if you want me to send it to you!
This resort is high up on my list of possibilities. Would you be able to email me the price list of extras? email@example.com