I didn't trot out Sister Wendy to appease Catholics. I'm a fan of her meditations and spirituality so i reached for her interpretation. There are dozens of other highly religious critics that have tackled it if you consider hers substandard because she started public criticism late in life.
As well - you says it's a literal act with specific meaning. He literally did not urinate on it. The urine was carefully collected and lit a very specific way in an attempt to look beautiful. See above about literalism and single avenue interpretations.
“... The urine was carefully collected.” Please... Enough already...