Solved it...
The solution is quite simple, you just have to define the ES query in the Metadata Provider. Here's a sample:
data:
lt_es_query type /plmb/t_spi_es_query,
ls_node type /plmb/s_spi_metadata_node,
ls_es_query type /plmb/s_spi_es_query.
... define your node specific attributes, then...
"Enterprise Search Query
ls_es_query-query_name = "query name, if used together with a standard application, should start with a Y/Z
ls_es_query-definition-template_id = "template ID found in TCode ESH_COCKPIT, example: 'PLMWUI~STANDARD~DOCUM_INFO_REC~'.
ls_es_query-definition-request_type_id = "request name, also found in ESH_COCKPIT, or 'DEFAULT'
insert ls_es_query into table lt_es_query[].
ls_node-queries-es[] = lt_es_query[].
insert ls_node into table mt_nodes[].