Is there some known possibility to pass user consent (especially on opt-out mode) on content which is nested via amp-iframe?
The iframe content itself contains an common html page so there is no way to use the "block-data-on-consent" attribute.
Maybe there is even an messy way to solve this issue without tweaking too much around.
You can embed an amp-iframe within your consent dialog which can send a postMessage with the consent result back to the amp-consent component. See the amp reference for details.