One approach would be to set a property and then
key on that with the logic:present / logic:notPresent tags.
Though, it is often cleaner to just have two pages, and
then select one or the other at the controller level. The
chrome surrounding a form can be shared using the
Struts template tags or

Tiles
.

If you were thinking of a tag that would either do the
equivalent of bean:write or html:text, it probably
won't show up in the taglibs distributed with Struts.
Though, there is an active extension community,
including packages like

struts-layout

which does similar things. I could see an "readonly"
property being added to their field tag, where you
could specify a property in the form bean that
returned true or falses.

I agree that taglibs like layouts are useful, but right
now the Struts team is waiting for the JSTL to finalize,
and looking forward to other technologies like
JavaServer Faces.

HTH -Ted