I can see a few ways of doing this:
- Round robin DNS with multiple A records (as suggested in comments by JFL). This would require the clients to properly re-look up the addresses when required, and get rotated A records (small TTL would probably help) and/or properly try the different addresses itself.
- VRRP/HSRP (per your original question) This looks like a perfectly good way
- Anycast with multiple NTP servers, perhaps with SLA tracking to decide which one to use
- Unsolicited Broadcast client where client just listens for broadcast time, and you have multiple broadcasters
- SNTP Broadcast client might be also suitable, where the client has a broadcast address as its "server", and accepts whatever answers come.
All of these will have the same condition that the far server, as seen by the client, might change to a different server altogether. This means that you should expect trouble if you were using peer-to-peer rather than client-serve modes of NTP. Given the description of your devices, I'd guess they are simple clients anyways.
I'd suggest the VRRP/HSRP and anycast solutions are going to be better than DNS round-robin because they don't require anything of the client. Choosing between these two would depend on your other networking criteria.
If the clients can support it, you might also look at making them broadcast clients or multicast clients. Rereading RFC 1769 SNTP it looks like at least one of the broadcast modes would specifically cover what you're trying to do, and might be lots simpler than getting clever with the router.