I wanted to ask if it's possible to use a var attribute inside an a4j:repeat tag, as a binding of an inner componente. For example
<a4j:repeat value="#{myController.bindingComponents}" var="component">
<h:panelGroup binding="#{component}"/>
</a4j:repeat>
I tried this already, however component is null when the binding expression is evaluated.
No, that's not possible. The binding attribute is evaluated during view build time (like the id attribute and all tag handlers), while the value of <a4j:repeat> as being an UI component is evaluated during view render time. So at the moment the binding attribute evaluates, the #{component} is null, because the <a4j:repeat> hasn't run at that point.
If you think once more about it, it should make completely sense: there's only one <h:panelGroup> in the JSF component tree which renders its output multiple times. It's not true that multiple <h:panelGroup> components will be generated this way, as you seemed to expect.
To achieve the particular functional requirement you've had in mind, you probably want to use a view build time iterating tag instead, like JSTL <c:forEach>.
