The lack of DWCAS as a primitive in general is really annoying, C++ aside. RISC-V's -A extension has no form of it, either; you only get XLEN-sized AMO + LL/SC (no 2*XLEN LL/SC either!) I was going to try porting an old lock-free slab allocator to my hand-written RISC-V core, and it uses DWCAS in precisely the way you mention. Immediate roadblock.

It's one of those features that when you want it, you really really really want it, and the substitutions are all pretty bad in comparison.