the netsuite field explorer chrome plugin even shows it filled

Hi Menno. This is Suitescript 1.0 unfortunately, but might give you a head start:

var filters = new Array();
filters[0] = new nlobjSearchFilter('internalid', 'customerprimary', 'anyOf', customerId);

var columns = new Array();
	columns[0] = new nlobjSearchColumn('internalid');

results = nlapiSearchRecord('contact', null, filters, columns);