Yeah, so in Android Components we already have some components that can react to low memory situations and we let the app tell us when this is the case, e.g. see:
For "engine" implementations we already support that a "tab" can either have an engine session (e.g. GeckoSession) or an engine state (e.g. GeckoEngineSessionState) attached. When needed we will restore the state to get an actual session (e.g. when displaying a tab). For example when we restore the browser state from disk initially all tabs will only have their state attached and we will not create a session for them until needed.
We haven't implemented memory saving capabilities for the engine yet, but we were planning to do something like that: In a low memory situation get the latest state and attach it to the tab, then close and free the engine session (Gecko Session). Do this either for all tabs but the selected tab or use some "last recently used" heuristic.