How can I populate Liquibase parameter values from an external properties file?

Do that at compile time: sounds like job for maven filters and/or profiles

NOTE: be carefull with liquibase and any "marker" replacements... liquibase stores CRC of applied changessets

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>test</groupId>
    <artifactId>test</artifactId>
    <version>0.0.1-SNAPSHOT</version>

    <build>
        <filters>
            <filter>src/main/filters/liquibase.properties</filter>
        </filters>
        <resources>
            <resource>
                <directory>src/main/resources</directory>
                <filtering>true</filtering>
                <includes>
                    <include>liquibase.xml</include>
                </includes>
            </resource>
        </resources>
    </build>
</project>

/src/main/filters/liquibase.properties

table.name=TABLE_NAME
column1.name=COLUMN1_NAME
column1.length=10
column2.name=COLUMN2_NAME

/src/main/resources/liquibase.xml

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<databaseChangeLog logicalFilePath="liquibase.xml" xmlns="http://www.liquibase.org/xml/ns/dbchangelog" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.liquibase.org/xml/ns/dbchangelog http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-2.0.xsd">

    <changeSet author="me" id="changeSetId1">
        <comment>Test</comment>

        <createTable tableName="${table.name}">
            <column name="id" type="int" />
            <column name="${column1.name}" type="varchar(${column1.length})" />
            <column name="${column2.name}" type="int" />
        </createTable>
    </changeSet>

</databaseChangeLog>

EDIT: A typical invocation (using filtered resources) would look like this: mvn resources:resources liquibase:update or more preferably use profiles... mvn resources:resources liquibase:update -P<profile_name>

EDIT2: There is one big advantage of this way of defining columns. You could use this property's (e.g.: column1.length) value (e.g.: 10) for validation of every layer: Hibernate, DAO, WEB, faces, JavaScript. Just use this property at each place where you need to validate against it. Even in i18n/messages.properties if needed (e.g.: input1.validation=No more than ${column1.length} letters.).

The only complication is that if you need to change this value you need to provide proper liquibase update/rollback script. Sometimes it is possible to change value and set new liquibase checksum (safe operation like increase varchar length), but other times you need to create a safe update changescript using new property/value.