Throughout the history of the church though there are stories of saints being visited by saints, having conversations about spiritual things and in the case of Thomas Aquinas even being taught about difficult spiritual matters. There is also the moment on the the mount of transfiguration when Jesus along with Peter, James, and John were accompanied by Moses and Elijah long since dead before that time. I know that in Matthew 22:32 Jesus mentions that God is the God of the living, and Paul writes that those believers who do die here are alive and well with Jesus. Would it be safe to say that these believers are momentarily given sight into and even caught up in the spirit and are temporarily granted access to the spiritual realm?
I can’t comment on events regarding Catholic traditions, but there are a number of occasions in the Bible where spiritual beings, be they angels, deceased humans, or the Lord Himself, have visited the physical realm as Moses and Elijah did in your example, just as there are descriptions of mortals visiting the spiritual realm, as in the cases of Paul (2 Cor. 12:1-4) and John. (Rev. 4:1-2) In each of these cases, permission was granted by God for the edification of mankind. Unauthorized communication between the realms is forbidden.