After giving it some further thought, I tried the following (which seems
more logical) but that doesn't work either...

*** ctx-test.tex ***

\starttext

\doifmode{mode1a}
    {mode1a}

\doifmode{mode2a}
    {\blank mode2a}

\stoptext

*** ctx-main.ctx ***

<?xml version='1.0' standalone='yes'?>

<ctx:job>
     <ctx:preprocess/>
    <ctx:flags>
            <ctx:include name='ctx-flags.ctx'/>
    </ctx:flags>
    <ctx:process>
        <ctx:resources>
            <ctx:include name='ctx-modes1.ctx'/>
        </ctx:resources>
    </ctx:process>
     <ctx:postprocess/>
</ctx:job>

*** ctx-flags.ctx ***

<?xml version='1.0' standalone='yes'?>

<ctx:flag>texengine=luatex</ctx:flag>
<ctx:flag>interface=en</ctx:flag>
<ctx:flag>result=ctx-modes</ctx:flag>

*** ctx-modes1.ctx ***

<?xml version='1.0' standalone='yes'?>

<ctx:mode>mode1a</ctx:mode>

*** ctx-modes2.ctx ***

<?xml version='1.0' standalone='yes'?>

<ctx:mode>mode2a</ctx:mode>

*** end of code ***

A Google search for the XML include element didn't bring much wisdom either.
What's missing/wrong please ?

-- 
Best,
Alan

* texexec --lua foo
* ConTeXt  minimals ver: 2008.10.31 13:58 MKIV  fmt: 2008.10.31  int:
english/english
* Ubuntu 8.04 Hardy Heron