It should be possible to specify a n_val when creating a Yokozuna index via the client library. This option is available via HTTP as of Riak 2.0.5. Moreover, if not explicitly specified, a default value will be used (buckets.default.n_val property), which may not match the destination bucket or bucket type.

Example:

Let's say the default value is 1, that is buckets.default.n_val = 1 in /etc/riak/riak.conf

And we create a bucket type with n_val = 3
riak-admin bucket-type create n_val_3 '{ "props": { "n_val": 3 }}'
riak-admin bucket-type activate n_val_3

Trying to create an index (named test_index in this example) and associate it with a bucket of this type results in
Invalid bucket properties: [{n_val,<<"Bucket n_val 3 must match the associated search_index test_index n_val 1">>}]