I had the same question and ended up figuring it out by reading the multi_index.hpp from eosio.cdt code and wanted to share my solution.

Instead of using auto, declare the type by using myindex::const_iterator:

struct [[eosio::table]] _obj {
    name key;
    uint64_t primary_key() const { return key.value; }
};

typedef eosio::multi_index<name("myindex"), _obj> myindex;

void do_biz_stuff(myindex::const_iterator& o) { ... }

Additional Note

If you want to use modify, erase, or iterator_to inside do_biz_stuff you must be sure to pass the iterator by reference and also to pass the multi_index instance by reference. Otherwise you'll receive the errors like object passed to modify / erase / iterator_to is not in multi_index or copy constructor of 'item_ptr' is implicitly deleted because field '_item' has a deleted copy constructor

void do_biz_stuff(myindex& idx, myindex::const_iterator& itr) {
    idx.erase(itr);
}

I had the same question and ended up figuring it out by reading the multi_index.hpp from eosio.cdt code and wanted to share my solution.

Instead of using auto, declare the type by using myindex::const_iterator:

struct [[eosio::table]] _obj {
    name key;
    uint64_t primary_key() const { return key.value; }
};

typedef eosio::multi_index<name("myindex"), _obj> myindex;

void do_biz_stuff(myindex::const_iterator& o) { ... }

Additional Note

If you want to use modify, erase, or iterator_to inside do_biz_stuff you must be sure to pass the iterator by reference and also to pass the multi_index instance by reference. Otherwise you'll receive the errors like object passed to modify / erase / iterator_to is not in multi_index or copy constructor of 'item_ptr' is implicitly deleted because field '_item' has a deleted copy constructor

void do_biz_stuff(myindex& idx, myindex::const_iterator& itr) {
    idx.erase(itr);
}

Using with Secondary Index

Also I just ran into the usecase of using with secondary index.

Define template to pass types:

struct [[eosio::table]] _obj {
    name key;
    name secondary;
    uint64_t primary_key() const { return key.value; }
    uint64_t mysecondary() const { return secondary.value; }
};

typedef eosio::multi_index<
   name("myindex"), 
   _obj,
   indexed_by<name("mysecondary"), const_mem_fun<_obj, uint64_t, &_obj::mysecondary>>
> myindex;

template <typename idxT, typename itrT>
void do_biz_stuff(idxT& idx, itrT& itr) { ... }

Usage:

myindex myidx(_self, _self.value);
auto mySecIdx = myidx.get_index<name("mysecondary")>();
auto mySecItr = mySecIdx.lower_bound(name("testSecondaryKey"));

do_biz_stuff<decltype(mySecIdx), decltype(mySecItr)>(mySecIdx, mySecItr);