It can be done, but only for editing text. You won't be able to edit HTML this way. Everything you enter will be HTML escaped. It would still be useful when you wish to quickly change the text of an element though.
The reason why HTML editing isn't allowed in the HTML panel (and also why importing HTML files results in a Custom Code component) is that it is impossible to recognize what Bootstrap components are used only from looking at the HTML code. It can be done in some cases, but it won't be reliable. And nearly everything that Bootstrap Studio does depends on the app knowing what components are used on your pages.
We work around this limitation with the Custom Code component - Bootstrap Studio ignores its contents so you can write whatever code you wish there. But you lose all the smart features that set Bootstrap Studio apart from other apps.