Hi Steven,
Thanks for the reply - understood there's some customization already possible, I am just looking for a bit more

Example: in the now playing panel, I'd like per-line size/color control for the data, eg, make the song title 12pt bold yellow, artist & album 10pt white, genre 8 pt white, or etc etc. There does not appear to be that level of control. In fact, for the first line it seems always to be bolded as long as it's just <Title> but otherwise not (even if it's something like: <IsNull(<Title>,NO TITLE!,<Title>)>. And the font size is fixed to one size for all lines.
Another (esoteric) example: I have a custom tag "<swearing>" so I can create auto playlists that don't have excessive cursing in them when the kids are around

In f2k I had a conditional line shown in red (Swearing!!) if that tag exists - don't see a way to do that in mb (I have the line there but it's the same font & color as the rest, it doesn't jump out).
Re: the notification dialog, same kind of thing, there's some amount of customization, just looking for the next level. Eg, you can change the font & font size but not the font color. And you're limited to just one tag displayed per line and no conditional stuff (ifnulls & such). On a side note, would also be nice if this popup "auto-scaled" to the longest line, up to a max width threshold (an option available in toaster for f2k).
Here's the kind of thing I was doing in f2k for now playing info:
'[size=14]
'$if(%artist%,%artist%,'<'No Artist'>')'[/size]'$crlf()
$if(%performer%,'[size=12]
'%performer%'[/size]'$crlf(),)
'[size=14]
'$if(%title%,"%title%",'<'No Title'>') %length%'[/size]'$crlf()
'[size=12]
'$if(%album%,%album% ,'<'No Album'>')$if(%discnumber%,',' Disc %discnumber% of %totaldiscs% ,' ')'[/size]'$crlf()
'[size=12]
'$if($stricmp(%album artist%,%artist%),,%album artist%$crlf())'[/size]'
'[size=12]
'$if(%genre%,%genre% )$if(%date%,'('%date%')',)'''[/size]'$if($or([%genre%][%date%]),$crlf(),)
'[size=12]
'$ifequal(1,%swearing%,Swearing!$crlf(),)'''[/size]'
$if(%tracknumber%,Track $num(%tracknumber%,0)', ',)$if(%tracknumber%,$ifequal(%play_count%,0,never played,)$ifequal(%play_count%,1,played %play_count% time,)$ifgreater(%play_count%,1,played %play_count% times,),$ifequal(%play_count%,0,Never played,)$ifequal(%play_count%,1,Played %play_count% time,)$ifgreater(%play_count%,1,Played %play_count% times,)) $if(%isplaying%,'('e: %playback_time% | r: -%playback_time_remaining%')',)$crlf()
$if(%last_played%,Last played on $select($substr($date(%last_played%),6,7),Jan,Feb,Mar,Apr,May,Jun,Jul,Aug,Sep,Oct,Nov,Dec) $if($greater($substr($date(%last_played%),9,10),9),$substr($date(%last_played%),9,10),$substr($date(%last_played%),10,10))', '$substr($date(%last_played%),0,4) at $select($substr($time(%last_played%),0,2),1,2,3,4,5,6,7,8,9,10,11,12,1,2,3,4,5,6,7,8,9,10,11,12)$substr($time(%last_played%),3,5) $if($greater($substr($time(%last_played%),0,2),11),pm,am)$crlf(),)
'[size=11]''
'%bitrate%Kbps %codec%, %channels%'''[/size]'