I don't think it's at all tacky asking for money. If you are worried about it being a little impersonal I would suggest telling people what you are putting the money towards. For eg, my friends just got married overseas and so they wouldn't have to bring back a stack of pressies they asked everyone for money 'to go towards their honeymoon.'

I know it's not the same as a wedding, but for my DP's 30th bday, we asked everyone to donate towards a new BBQ that he wanted (we held a function with a 3-course meal, entertainment and drinks at a renovated pub). Even ppl who couldn't come sent us a small amount and we ended up getting as much as the whole night cost us. Afterwards, we bought the BBQ, took a photo and sent it to everyone in a thank you card. Maybe you could do something like that?