I'm wondering about a similar thing. As I'm doing a Multiplayer game I would like to load character files on demand as they log in, and unload character assets as they log out. Would this be possible?
You cannot unload, except when changing layouts.
Memory is managed by the browser engine, C2 has no control over it besides the initial drawing of an asset (first time will load it into memory).
It's the downside of the engine, that its suitable and streamlined for game development to keep these advanced features hidden, but it detracts from power-users who prefer to have this explicit control.
This matters but it's becoming less of a problem as mobile devices advance and they have a lot of memory.
Once you become adept at using C2, you have to design around it's limitations if you want to make a grand-scope games.
The funny thing is Unity also behaves like this, I hear from Steam devs who complaint about their lack of control over memory management.
Even turn based games, hex grid, like Thea the Awakening, lags like hell in some parts due to Unity not behaving.