It is indeed possible and I’ve done it with other routers. These have a setting to explicitly run radvd in a “router only” mode, so no prefixes are advertised.
ICMPv6 works fine as long as the router has a GUA on at least one interface. It is not required that every interface has a GUA.
And this is just one use case. Another one is a network where addresses are assigned via DHCPv6 (exclusively, no SLAAC) and all traffic should be routed (no direct communication between hosts). In this case you don’t advertise a prefix either.
RouterOS seems to have a somewhat weird behaviour when it comes to sending RAs. When there is no prefix to advertise on an interface, it stops sending RAs from this interface entirely.
My current workaround is to add a dummy prefix with disabled on-link and autonomous flags. RouterOS then sends RAs with this (meaningless) prefix. This is rather hackish though, there needs to be a better solution.