It seems like it's tough to get much further along this line without knowing, first, who the slavers are and, second, their reason for taking the party members (spellcasters especially) as slaves. To the traditional "put them to work in the fields" slaves, I can't see why they would not maim the mage/cleric to prevent spellcasting entirely, or just kill them (ie not retain spellcasting slaves), which seems unlikely to make for a fun game. What in the game requires the PC's be taken as slaves, for how long, and who are the slavers?
I'm still leaning to careful supervision and rest/prayer/meditation interruption (even that Sorcerer needs 15 minutes of meditation every morning), but the question of cantrips is one that bears review. If I don't spend the time to regain my spells, do I still have my cantrips? I'd probably say yes, as I believe you also still have any spells/slots you didn't expend yesterday, and you never fully "expend" cantrips.