I am wondering this myself as well. Although as per official guide line, it seems stock is counted, quite a few veterans here suggested that it's better to have cash (i.e. sell the stocks to increase the cash balance, and attach some reference to explain the balance increase). This is because some office might not recognize stock investment as acceptable POF, as it's volatile and the actual value is therefore undetermined.
However selling stocks and get the cash into your bank account will take time (or at least that's the case for me), so I am quite concerned that the cap will run out (not to mention selling them right now might not be financially favourable). I guess really it's a risk that we have to take whether to proceed with submitting the stock investment portfolio as POF. For me I am monitoring the number of applications (checked via the google doc everyone is putting in here), if I feel that the cap is running out quickly, I'll just bite the bullet and take that risk. Let me know what you will do. All the best