The solution now looks rather simple, but I had headaches first trying out all sorts of BBCode > HTML, and regex stuff that sometimes completely failed, sometimes seemed to work, but only under some circumstances, or even fatally crashed BBCe.
It's really a pity there is no help file, nor a tutorial on how this expression/replacement button stuff is supposed to work exactly.
I couldn't even figure out the purpose, workings and differences between Command > Single tag (i.e. hr) and Double tag (i.e. code).
These smart coders should really make an effort sometimes to place themselves in the minds of mortal end-users...
In case other users dare to explore this expression/replacement functionality, perhaps it is of some use for me to share why (I think) many of my earlier attempts failed.
I started with entering complete HTML replacement strings into the replacement panel.
Such as e.g.: <span style="font-size:4px;"> </span><span style="font-size:20px;">$1</span>
(and dozens of variations on that :-(
But for the purpose of these particular buttons, no HTML code seems to be needed at all.
It seems to only replace parts within the HTML code that the preview panel is already using.
I'm quite a bit out of my depth here, but maybe this provides some insight to others attempting to use this functionality.
And then come up with some other useful button ideas?...