Although many protestant churches today may be viewed as being a little more relaxed on closed communion, I think that is a generalization. More specifically it may apply to nondenominational, non-liturgical, and non-Sacramental protestant churches. But, having been an active LCMS member for 57 years before my conversion to, which is a very liturgical and sacramental denomination, I believe one would be extremely hard pressed to find a LCMS church that would welcome non Lutherans to communion. I agree, there may be some flexibility with other branches of the Lutherans, but even so, my experience tells me doing so would not be broadcast, and would cause quite a stir in the very conservative LCMS.
I, out of respect for their beliefs, would not (as a Catholic) even ask to receive communion at an LCMS church, just as I would not think it appropriate for a non-Catholic to receive the blessed sacrament at my parish.
Although, how do you tell what denomination someone is who is receiving communion. In a Catholic Church, its easy…you see it a lot on Easter and Christmas, when there are visitors to the parish…the tip off is the exchange between the Priest and the person receiving the Eucharist. If, when father says, “The body (or blood) of Christ,” and the answer is not, “Amen,” but, “Thank you, Father,” you can bet the visitor not only does not know of our reverence of the Sacrament, but is also not a Catholic.
Whenever I invite non-Catholic friends to Mass, I explain the sanctity of the sacrament, in a way that does not make them feel they are being arbitrarily excluded. Instead I keep it positive, because it is an excellent moment for catechesis…Hopefully all parishes welcome visitors to come before the table of the Lord in solidarity, to receive a blessing from the priest or Eucharistic Minister, even if they are not able to join in full communion with the Church.
Pax