I'll say it right now: OpenTimestamps could absolutely work with something unlike Bitcoin underneath. Hell, trusted time-stamping with it (specifically with one-time-use disposable keys) is on my to-do list. And widely-witnessed time-stamping has already been implemented.

Single use seals also could be implemented in a variety of ways. That's the whole point of coming up with the idea: to separate the cryptographical primitive from the implementation.