I like the new order. "{register}<command> is difficult to reach. <command>{register} is much easier.
How about mapping them to z and Z?
map global normal -docstring 'Restore selections from register' z ': restore-selections<ret>'
map global normal -docstring 'Save selections to register' Z ': save-selections<ret>'
Hmm I didn’t realise I switched the oreder :0 , is not very orthodox !
If I understand you well, I see one problem : you will loose the default register feature: pressing z or Z is a shortcut for "^z or "^Z respectively.
So it’s a tread off : having to always press 2 keystrokes ( z + {register} ) , or having to press 1 keystroke in most cases and 3 keystoke from time to time ( z or "{register}z )
I guess it depend on how you use it.
In the first case you can also use z as a register, so you have to press zz which is close to pressing one keystroke.