No idea on the problem itself because I don't understand it, just:

Given that, we either need to address the const requirement
for userspace, alter userspace's usage, or find another way to address
the deadlock.  Any ideas?

In the worst case you can always disable the vsyscall gtod() and
thus the need for a user space safe seqlock_t, but it will
hurt some workloads significantly. 

-Andi