Is it possible to use

Answer by Alessandro A. Garbagnati

No, unfortunately it is not possible. The <rtexprvalue> tag in the tag library descriptor allows only expressions to be evaluated, but not another custom tag.
The only way (and I know this is not a solution but just a 'bad' patch) is to pass these type of information throught the attributes.
Answer by Luigi Viggiano

I know this code surely works:

<jsp:setProperty name="aBean" property="aProperty" value="<%= aValue %>" />

Extending the idea... this should be ok:

<mylib:mytag key="myKey" arg0="<%= aBean.getAProperty() %>" /> 

Try, and let me know.