Hokie Pundit laments the fact that some branches of Christianity do not have open Communion and that some get unduly hung up on the use or non-use of alcohol for Communion. An ex-priest I know (as well as a very close loved one) also think the RCC's Communion policy distasteful. Hence this question interested me.
I think CS Lewis would beg to differ. He urged no one to stand in the hallway of Christianity, but to pick a room (i.e. denomination or branch) and live its tenets and particularities. To have an open Communion, it seems you'd have to have it in the hallway, metaphorically-speaking.
There are some things even the Pope has no power to change - such as the use of wine in Communion. What is special about wine? Or what is special about water, when used in Baptism? Besides that Jesus used both, there's a sense in which water, for example, is not merely a symbol of cleansing but was created firstly for Baptism and only secondarily for thirst-quenching and cleansing. In other words, instead of thinking that God appropriated water as a symbol since it had cleansing and thirst-quenching properties, consider that He imagined primarily for the sacrament and that secondary uses were applied so that its real use in Baptism might be better understood.
For those who think, "who cares? it's just a material substance", think about the universe. That an invisible God created a material universe leaves us wondering why, but the fact that he did makes it, by default, important. The fact that God-made-man decided to attach an importance to common everyday objects is determinative, because God alone determines whether something is important.
The thing not too many people like to bring up is that Catholics believe, or are supposed to believe, that Communion is something entirely different from what an evangelical would believe it to be. Thus I'm not sure how you can have an "open Communion" when the very thing itself is the object of dispute.