Same problem on 5-1 branche. This is what I do as a test.
In .ADP:
<master>
<property name="title">@page_title@</property>
<formtemplate id="html_area"></formtemplate>
In .TCL:
set page_title "Test HTMLarea"
template::form create html_area
template::element create html_area referer \
-optional \
-value "/" \
-datatype text \
-widget hidden
template::element create html_area content \
-value "{Hello HTMLarea} text/html" \
-datatype richtext \
-widget richtext \
-htmlarea_p "t"
