Hi Miklos
I would say you're on the right track. Yes a hidden field will get your 'is-valid' persisted and therefore available to be included on the Summary screen.
The validity of the form is held within the fr-error-summary-model within the fr-error-summary-instance as a boolean within the 'valid' element.
So if you had a control 'is-valid' within a hidden section 'hidden-section' you could include an action that populates the control and runs on save:
<xf:action ev:event="fr-data-save-prepare" ev:observer="fr-form-model" if="true()">
<xf:setvalue ref="instance('fr-form-instance')/hidden-section/is-valid" value="xxf:instance('fr-error-summary-instance')/valid"/>
</xf:action>
Disclaimer: I don't know the roadmap for fr-error-summary-instance, so can't say whether this is supported going forward.
Regards
Jez