I figured out the string replace...
For anyone who's interested, I added the following code:
sayregexp "(?<=\"artworkUrl100\":\").+?(?=\")" ", " "}"
regexpreplace "100x100" "600x600"
just before:
outputto "COVERURL"
sayregexp "(?<=\"artworkUrl100\":\").+?(?=\")" ", " "}"
Hope that helps someone...