If I were one of the guests and got 40 days notice of the cancelled wedding, the only money I would be out was the cancellation fee on plane tickets, IF THAT. If I was a local, I wouldn't even have that expense. If I am close enough to someone to be willing to buy plane tickets to their out of town wedding, then I'm close enough to them to take the financial hit of a cancellation fee and never mention it to them, EVER. I would file it under "crap happens" and would feel terrible for them. After all, the bride and groom must know that some people are out some money. Unless they are immensely self involved people, they probably ALREADY feel bad enough about people being out some money. They certainly don't need me to make them feel worse about it, at the hardest, most stressful time of their lives. I'd never mention it, and I'd certainly not act like I have any say in what they do with items already purchased for the wedding.
Let's face it, there is no PERFECT solution to make everyone whole in the case of a cancelled wedding. It's not an ideal situation. People are going to be inconvenienced, and people are going to lose some money. I don't see much way around that. I have heard of cases where the couple sells or returns the rings and uses the money to pay back bridesmaids and such, which I think is very classy and a good way to "own" the expense of the cancellation yourself instead of expecting guests to own it, but this may not always be an option and I do not think etiquette requires it. There may be no way to reimburse people for money already spent, and I honestly don't think most people would expect it. If this happened to a good friend of mine, I wouldn't concern myself with what I spent and would be surprised to receive any reimbursement of any kind.
Yes, some of the guests are "out" some money. But they did get enough notice to cancel other expenses and I'm betting they got a sincere apology for the inconvenience. Sometimes in life, that is all we get. We don't always get our money's worth out of every single transaction. I'd rather risk losing out on a cancellation fee than never go to any weddings.