You can just put the SearchBox inside the heading if you're define a heading like
<div>
and not like header.
<div data-dojo-type="dojox.mobile.Heading"
data-dojo-props="label:'Label'">
<input id="searchBox" data-dojo-type="dojox.mobile.SearchBox"
data-dojo-props="type:'search'" type="search"
placeholder="Placeholder..."
style="position: absolute; margin-right: 6px; right: 0px; top: 15%; width: 30%;">
</div>