On the topic of pg_lsn, I recently noticed that there's no operator(+)(pg_lsn,bigint) nor is there an operator(-)(pg_lsn,bigint) so you can't compute offsets easily. We don't have a cast between pg_lsn and bigint because we don't expose an unsigned bigint type in SQL, so you can't work around it that way.
I may be missing the obvious, but I suggest (and will follow with a patch for) adding + and - operators for computing offsets. I was considering an age() function for it too, but I think it's best to force the user to be clear about what current LSN they want to compare with so I'll skip that.