I don't think it's cheeky as long as you're very upfront in the invitation and say no offence at all will be taken if people can't make it. At the end of the day, it should be the christening that is the important bit, not the party that follows so I don't see why you should wait to have your LO christened until you're better off. I personally don't understand why christenings are turning into such big events, it's sort of losing the purpose - people shouldn't be coming for the party. Maybe say that you don't want any gifts, just their presence on your LO's special day?