Really easy way, not the cleanest way to do this is:
Note: This applies for DITA-OT 1.8.4 check suitability for other versions.
Create your custom PDF plugin as told in: http://dita-ot.github.io/1.8/readme/dita2pdf-customization.html
Create "insertParameters.xml" as in: http://dita-ot.github.io/1.8/dev_ref/plugin-xsltparams.html
In my case with following custom parameters:
<?xml version='1.0' encoding='UTF-8'?>
<dummy>
<!-- EXAMPLE: <param name="paramNameinXSLT" expression="${antProperty}" if="antProperty"/> -->
<param name="custom.data1" expression="${custom.data1}" if="custom.data1"/>
<param name="custom.data2" expression="${custom.data2}" if="custom.data2"/>
</dummy>
Now drop this "insertParameters.xml" into DITA-OT\plugins\org.dita.pdf2
Create processing to custom.xsl under xsl\custom.xsl.
Here is an example code snippet for custom.xsl
<?xml version='1.0' encoding='UTF-8'?>
<snippet>
<xsl:param name="custom.data1"/>
<xsl:param name="custom.data2"/>
<fo:block><xsl:value-of select="$custom.data1"/></fo:block>
<fo:block><xsl:value-of select="$custom.data2"/></fo:block>
</snippet>
Run integrator.xml to integrate changes into your DITA-OT.
Give in command line commands for running your PDF scenario like: "ant dita2mypdf -f -Dcustom.data1="myCustomParameter1value" -Dcustom.data2="myCustomParameter2value"
Run the process to create PDF and your parameters should be visible!
What this question was really intended to achieve to get a step by step instruction to set all these things to work 100% in the custom plugin that not a single thing needs to be added into DITA-OT default plugins (not even single file into pdf2 folder as in this solution). This could make updating DITA-OT remarkably easier for future.