If it were me, I wouldn’t put a Dampit in my harp. I’ve seen pictures of a harp damaged by one – it ruined the base and the body shell, so I’d be cautious about putting anything damp/wet inside your harp. Plus it will only humidify the body chamber while the rest of the harp receives no benefit. 42% room humidity is acceptable, though on the low end of the target range of 40%-60%, with the outer limits ranging from 30%-70%.
Hope that eases your mind some.