Every epub reading app (on eink or Android) I've ever used which offers some kind of option to choose between left align (ragged right) or full-justified uses a broadly similar method to enforce the user's choice.
It's always a small piece of override CSS which looks something like one of the following (
in descending order of "aggressiveness"):
- * {text-align:left !important;} (or * {text-align:justify !important;}
- div, p {text-align:left !important;} (or div, p {text-align:justify !important;}
- body {text-align:left !important;} (or body {text-align:justify !important;}
The choice of which HTML tags to apply the override to is entirely up to the app developer. Whatever level they choose they will end up delighting some users and upsetting others. Some apps (e.g. KOReader) may give the user a limited choice of how aggressive to be.
If Lithium has opted for the most aggressive override, * {text-align:left !important;}, then there is nothing you can do to prevent your <h1> tagged centred text from being forced to text-align:left/justify if you press the left/justify option buttons. The only way to keep them centred is to leave the 'Text align' option set to 'Original'.