Not sure if I understood the question.. but essentially you will not be able to access nxt-assets from mynxt. Anyhow you are dealing always with the one same nxt-account which accessible with the same long passphrase (=brainwallet) no matter which service you choose to use to access that account (mynxt, secureAE, local client etc). Your nxt-assets (for example RSU asset) is included in that very account.

So again, mynxt can not access the asset-functionality of your nxt-account, but the nxt-assets are there. So you have to use secureAE or local client to buy, sell and transfer nxt-assets, for example RSU assets.