Don't know about the shoes, the explanaition about the Pope's slippers sounds feasible at least, but I do know that St Peter's finger, which features in at least one other song, (sorry, it's been driving me spare trying to remember which one) and about 6 different pubs in the UK called that. It comes from the latin St Peter ad vincular, anglisised to St Peter's vincular, which in turn became St Peter's vinger, or finger.... often called St Peter's digit.....

Wanting to be in someones shoes is taken as a meaning you wish you were them, so if you wished you were in Peter's shoes, I guess it means you wish you were in heaven, at the pearly gates...

LTS