A random 64-bit suffix for SLAAC is not directly possible with odhcp6c. If you don't provide an explicit IPv6 address via -i (random is not a valid parameter), it obtains the 64-bit suffix from your wan interface's link scope address, which is generally fe80::<EUI64>, where EUI64 is generated from the MAC address set for the wan interface: https://github.com/openwrt/odhcp6c/blob/master/src/ra.c#L364-L376
Patching odhcp6c to obtain a 64-bit suffix from /dev/urandom is extremely trivial (either tied to a new option or a special value for -i).
I, however, have no interest in making a github account nor submitting pull requests, but I can post the patch here if anyone is interested in submitting the PR.