[SalesForce] Problem with WHERE Clause in SOSL to search KnowledgeArticles

Yes, there is a way to do that:

FIND 'test' RETURNING KnowledgeArticleVersion(Id, Title, Summary WHERE PublishStatus='online' AND Language= 'your language')

You can add a condition on channel and data cateogry inside as it's done with Publish Status.

Looks like the answer is in this post. I had dismissed looking at the Dynamic SOSL hits in google. https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_dynamic_sosl.htm#snippet

Use Dynamic SOSL to Return Salesforce Knowledge Article Snippets To provide users with more context for articles in search results, use the SOSL WITH SNIPPET clause. Snippets make it easier for users to identify the content that they’re looking for when the search term isn’t included in the article summary field. For information about how snippets are generated, see WITH SNIPPET in the Force.com SOQL and SOSL Reference.

To use the SOSL WITH SNIPPET clause in a dynamic SOSL query at run time, use the Search.find method.

Search.SearchResults searchResults = Search.find(SOSL_search_string);

This example exercises a simple SOSL query string that includes a WITH SNIPPET clause. The example calls System.debug() to print the returned article titles and snippets. Your code would display the titles and snippets in a Web page.

Search.SearchResults searchResults = Search.find('FIND \'test\' IN ALL FIELDS RETURNING 
KnowledgeArticleVersion(id, title WHERE PublishStatus = \'Online\' AND Language = \'en_US\') WITH SNIPPET (target_length=120)');

List<Search.SearchResult> articlelist = searchResults.get('KnowledgeArticleVersion');

for (Search.SearchResult searchResult : articleList) { 
    KnowledgeArticleVersion article = (KnowledgeArticleVersion) searchResult.getSObject(); 
    System.debug(article.Title); 
    System.debug(searchResult.getSnippet()); 
}