Tricky one Briar. I should imagine that this is probably easier to arrange when dealing with your own family (and how about roping in the mother of the bride to broach this with her side of the family?) than when dealing with friends of the couple. I'm not sure how I'd feel if the mum of one of my friends phoned up on their behalf to ask me for money for them. I should think that we'd both be highly embarrassed. And I can't help feeling from some of your posts that you don't feel that comfortable with this idea yourself. How about, therefore, suggesting that you will explore the range of possibilities but that her Mum explores how people feel about such a request with regard to her side of the family, you with your son's and that they should be the ones to raise it with their own friends.
I do know that the only wedding I have attended where the last item on the wedding list was a request for money towards the honeymoon it went down like a lead balloon! Admittedly, this was in the northeast of scotland were we do perhaps tend to take a more old-fashioned view of these things:-) But a surprising number of guests (who openly discussed it at the wedding!) felt that at this was one of the few things the groom was traditionally expected to pay for and that if he couldn't afford to do so well it was cheeky to expect his wedding guests to do so instead.
Times do change and the codes of etiquette having kept up - the old reason for giving a couple presents and, in Europe more, money ie to help them set up their first home and in lieu of a dowry, no longer applies to most couples but along with this, weddings themselves have become more elaborate and can be expensive for guests even before buying a present comes into the equation by the time the new outfit has been bought, travel and often overnight accommodation has been paid for etc and I do wonder whether an outright request to help pay for the wedding expenses of the couple concerned could be a turn-off for some people who might as a result decide not to attend. This certainly was the case in the wedding I referred to above - it was in all other respects a pretty lavish affair and the attitude of friends of mine who were also invited but declined was that the couple should have planned within their own budget and that if they couldn't afford to pay for their own honeymoon they should have economised elsewhere.
And finally, the last wedding I attended where the couple (admittedly older than I assume your son and his fiance are) where in exactly the same position, they decided not to have wedding list or ask for gifts at all. They nominated their favourite charities and circulated this list with the wedding invitations and suggested that if guests wanted to mark the event in some way that they either make a donation direct to one of the charities listed or pop a donation into a big jar that would be placed on the bar for the purpose at the reception. It raised a fair amount on the night alone which was subsequently divided between the charities.
SM
PS If you are still feeling uncomfortable with the idea of asking guests for money then it really would be a good idea to discuss this with the coule sooner rather than later. Feeling that you have to do something you aren't personally comfortable with could take the shine of what should really be a joyous day for you. Share your anxieties with them now rather than later would be my advice. Also, the fact that she has asked you to do it suggests that your future daughter-in-law might possibly be feeling a bit embarassed about asking people to pay for the honeymoon herself and/or is wondering about how guests might react.