+1 Darryl
A custom html box is in your widget edit area for the main and profile pages, its just a custom widget that displays custom html code. You just drag the widget link from the top parked area to where you want to show it and then put code in there.
I have just set up a contribution page using custom pages and thats how you should do it. But if you really want to add it to the membership plugin then it does not hurt to try however like Darryl said it may parse the html but there is no integration either way and it will have to all be done manually.
Also dont forget to add your disclaimer under your subscription button that advises them in order to cancel the subscription they need to log into their paypal account and cancel the subscription via the subscription page and that you cannot cancel subscriptions.