Was trying to figure out how to do it, but haven't seen how to setup just a general if statement since only using spellcast for about a day now.
Code 
<spellcast> 
                        <set name = "BSTBasic" >                          
                                <main>Brethren Axe</main>
                                <sub>Aalak' Axe</sub>
                                <ammo></ammo>
                                <head>Mikinaak Helm</head>
                                <body>Kariey Haubert</body>
                                <hands>Karieyh Moufles</hands>
                                <legs>Karieyh Brayettes</legs>
                                <feet>Karieyh Sollerets</feet>
                                <neck>Chivalrous Chain</neck>
                                <waist>Dynamic Belt</waist>
                                <lear></lear>
                                <rear>Ghillie Earring</rear>
                                <lring>Heed Ring</lring>
                                <rring>Woodsman's Ring</rring>
                                <back>Vassal's Mantle</back>
                        </set>
                <if body="empty" sub="empty" main="empty" hands="empty" feet="empty" 
legs="empty" neck="empty" waist="empty" rear="empty" lring="empty" rring="empty" 
back="empty">
                  
                        <if mainjob="BST">
                                <equip set="BSTBasic" />
                        </if>
                </if>
</spellcast>