Check multiple properties are set and not blank (ant)

I'm in a situation that involves running an ant build with optional parameters that are always specified but but not always defined, like so

ant -DBUILD_ENVIRONMENT=test -Dusername_ext= -Dconf.dir_ext= -Dcgi-dir_ext=

If the parameters are not given values on the command line they will be by loading a .properties file. I have the following code that will check if the property isset and is not blank.

<if>
    <bool>
        <and>
            <isset property="username_ext"/>
            <not>
                <equals arg1="${username_ext}" arg2="" />
            </not>
        </and>
    </bool>
    <then>
        <property name="username" value="${username_ext}" />
    </then>
</if>
<property file="${BUILD_ENVIRONMENT}.properties" />

Since there are multiple properties it seems like I should write a target that will do the same actions for each property rather than repeat that code every time.

<antcall target="checkexists">
    <property name="propname" value="username"/>
    <property name="paramname" value="username_ext"/>
</antcall>
<antcall target="checkexists">
    <property name="propname" value="conf.dir"/>
    <property name="paramname" value="conf.dir_ext"/>
</antcall>

But AFAIK an antcall will not set a global property. How then can I write a target that will accept the name of a parameter it needs to check is set and is not blank, and then copy that in to a parameter that other targets can use?