Well, there is @nospecialize but I think it is not recursive, and it cannot be used for function calls only function definitions. So it would only help to mitigate the problem a little.
Creating a wrapper subtyping DenseArray with an SArray abstract non-parametric field could work, but probably the boxing of the SArray will allocate (the type of the wrapper would always the same, but it would delegate all the work to a stack-allocated SArray).