Define Real time?
If you mean is Mach3 real time, then no it is not, it currently runs in a 10 hrts main update loop, with an optional faster 40 hrts update loop for fast small code.
If you mean real time possible at the machine, well yes, in a way. For instance the External Motion Card made by Galil, has buffer that Mach can load, so the board is always moving through its commands in real time in relation to the machine.
What you wanting todo...... You can implement some of what you want in a plug in that has your own custom device context that you translate those things to G code or DDAs to send out to a motion board....... Your writing "Tablet" could really just end up being a type of "Jog Pendant", in which in the config for that tablet and asociated Jog type plugin that you would do, you could have the machine follow your X/Y motion from the tablet, Scaling the tablets space to the equiveent machine softlimits, and Velocitys of your stylist in moves could not be faster than your machine could move....etc.
good luck to you,