I think I have it. Ineed to implement another schema method that will get me the field. Something similar what it is done with the task validation params.
<enter>
<soapCall name="GetValidationParameters" service="xtk:workflow">
<param exprIn="[@workflow-id]" type="string"/>
<param exprIn="@taskIdentifier" type="string"/>
<param type="DOMElement" xpathOut="/ignored/parameters"/>
</soapCall>
<set value="" xpath="/ignored/@value"/>
</enter> I have created wrapper to the function above that additionally looked up for the checkbox value saved under the particular workflow activity and saved it in the parameters. Which I reused to display entire section in the taskApproval form.