I do not see this as such a big problem, because if you can put together the data you need, similar to the LCD's and only relavantes further, the string should not be too long.
For what I imagined, that would be:
- CPU load
- CPU temperature
- GPU load
- GPU temperature
- RAM occupancy
The string could look like this:
25;55;30;45;3545;
or:
CL25,CT55,GL30,GT45,R3545;
So only the most necessary joined together with separators.
That makes 36 characters in the constelation, that is 360 bits.
Maximum update at 9600 baud: 26 / s.
But that's not all that much, since such ads are rather crude and sluggish.
Apart from that, even the small Arduino Uno creates the 115000 baud.