Custom defined operators can be used just fine from C#. The names are auto-generated and are of the form op_<symbol...>
(see Overloaded Operator Names on MSDN).
For example
let (|?) a b = ...
would be available as op_BarQmark
.
However, as Mau points out in his comment, you will only be able to use it from C# as a method, not an operator.