That's already possible with the <keywords> configurations, I do this as 
following for my webapp with Freemarker templating:
<properties>
        <!-- Gettext -->
        <gettext.sourceLocale>en</gettext.sourceLocale>
        <gettext.keysFile>messages.pot</gettext.keysFile>
        <!--
        Add the keywords:
        _ = 1st param is singular form
        _p = plural, 1st param is singular, 2nd param is plural form
        _noop = 1st param is singular form, this one is a special-case
        @see http://www.gnu.org/software/gettext/manual/gettext.html#Special-cases
        Set extra options to the xgettext command (without the backslashes!):
        \-\-no-location = do not write '#: filename:line' lines in PO files
        \-\-add-comments[=TAG] = place comment block with TAG (or those preceding keyword lines) in PO file
        -->
        <gettext.keywords>-k_ -k_p:1,2 -k_noop --no-location --add-comments=///</gettext.keywords>
        <gettext.poDirectory>${project.build.resourceDirectory}/locale</gettext.poDirectory>
    </properties>

     <build>
        <plugins>
            <plugin>
                    <!-- http://code.google.com/p/gettext-commons/
                         http://gettext-commons.googlecode.com/svn/maven2-plugins-site/index.html
                    -->
                    <groupId>org.xnap.commons</groupId>
                    <artifactId>maven-gettext-plugin</artifactId>
                    <version>${gettext.version}</version>
                    <configuration>
                        <targetBundle>Messages</targetBundle>
                        <sourceDirectory>${project.basedir}/src</sourceDirectory>
                        <sourceLocale>${gettext.sourceLocale}</sourceLocale>
                        <extraSourceFiles>
                            <!-- Use build specific templates directory -->
                            <directory>${freemarker.templates.dir}</directory>
                            <includes>
                                <!-- Include FreeMarker templates -->
                                <include>**/*.ftl</include>
                            </includes>
                        </extraSourceFiles>
                        <keysFile>${gettext.keysFile}</keysFile>
                        <keywords>${gettext.keywords}</keywords>
                        <poDirectory>${gettext.poDirectory}</poDirectory>
                    </configuration>
                    <executions>
                        <execution>
                            <id>merge</id>
                            <phase>generate-sources</phase>
                            <goals>
                                <goal>merge</goal>
                            </goals>
                        </execution>
                        <execution>
                            <id>dist</id>
                            <phase>compile</phase>
                            <goals>
                                <goal>dist</goal>
                            </goals>
                        </execution>
                    </executions>
                </plugin>
            </plugins>
     </build>

Awesome, this is helping me tremendously.

Would you be so kind to show some examples in your freemarker templates, 
ideally including a case with plural text? Much appreciated! 

Hank