See the related bug that should be handled in the Math code (bug 66278). It seems that at the moment, LibreOffice Writer only allows "inline" formulas. It should be possible to add an UI like
Format -> Formula -> Display/Inline mode
Right click on a Formula -> Display/Inline mode
so that the user can format an equation in "display" mode. This would have at least the effect of centering the equation in its own paragraph and transmit that info to Math so that the math rendering engine could take that into account. Then when exporting to XHTML the <math> element will have a display="block" attribute attached to it (I guess the XHTML exports relies on the MathML export and so this last point will be fixed in bug 66278) so that browsers can correctly display the formula.
Created attachment 81591 [details]
Sample odt document
Created attachment 81592 [details]
Here is an XHTML example that shows how the document should be exported to XHTML. Note the display="block" on the second equation and the different rendering when you open in e.g. Firefox.
OK, there is actually already an option Format => Text in Math to do that. It just remains to modify the MathML export to add the display attribute when necessary.