just looked into this and can confirm, jam cannot join a meet "natively".
but it is close enough. just use the google meet app from the iPad, and share the iPad screen from within Meet. it's the method i outlined, just with the jam app instead of 'Coloring'.
even without google meet on the iPad, the share-to-yourself and then share screen is unnecessary. create a new jam, join that jam from laptop and iPad, and share the jam from your laptop. this seems easier to me, but ymmv. https://allthings.how/how-to-use-jamboard-in-google-meet/
should also work with any other videoconf app. i assume they all allow screen share.
i really enjoy this method much better, because now in Catalina the screen recording permission is a distinct capability. I only enable that for Chrome when I explicitly need to share the screen. Setting those kinds of capabilities is not automatable, so it's a self-imposed annoyance, but worth it for the security gain. People really underappreciate the improved security of Catalina, perhaps justifiably so due to the other craptasticness of it. When you enable this in prefs, it says you have to restart chrome for it to take effect. However, you don't. The permission takes effect dynamically. Probably because each tab is a new process in Chrome, so it is in effect "restarting".
for vc apps that have their own app, this is great compartmentalization for the screen recording permission. for vc that runs in the browser, the tradeoff is you have to give the entire browser screen recording permission. this is too vast of a permission. of course with a native app you then have the zoom-malware problem. sigh.