We've been working on Collusion for a while now, which might do what you're talking about: shared collaborative white boarding (with a really big space - from a URL, ... with pen support, good for tablets, phones etc. We're testing so feel free to grab an account gratis - constructive feedback welcome: heres an example project https://col.lu/PSnZMIH and if you want an account to mess with: https://collusionapp.com