Yes, soaking could easily be species-specific. I can't speak for BPs on that- just generalizing what might help. But swimming motions works on all kinds for constipation, so maybe on slugs?
Your call for vet apptmt- it's very hard (or impossible) to palpate for slugs- I've triedbut they're too soft & squishy, & you don't want to hurt your snake with excessive pressure. You might call & ask the vet about a scan?