I know very well what you say about keyspan cards, they are rare an expensive. I have mine on my G4 MDD and it does the work perfectly with OMS, freemidi, etc etc. The main advantage over built-in serial on powerbooks is that you get two ports, so you can make a massive midi setup.
Anyway, as you use Studio5, you can stack up to three Studio5 on a single serial port, what means 45 separate MIDI lines, and make use of the awesome MIDI routing capabilities (studio patches).
What¡¯s the last, best Mac laptop with a serial port? -> powerbook g3 PDQ 300 Mhz; there are different clock speed processors options, but that is not essential as you have noted. Later models (those with Firewire/USB connectivity) does not have the serial port connector.?
What¡¯s the biggest screen in Mac laptop with a serial port? -> 14.1" . You can check specs on link above.
What¡¯s the last best tower with a serial port? -> Macintosh G3 desktop.?
The advantage on this model is that internal hard drive is IDE, instead of SCSI.
About SSD, some models work, others not. The HD on powerbooks are standard IDE 40pin 2.5", small form factor usual on laptops, so it?s not so hard to find mechanical replacements.?
:)
Jos¨¦ Juan